feat(lsp): support textDocument/onTypeFormatting (#34637)

Implements [on-type
formatting](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.18/specification/#textDocument_onTypeFormatting)
using a `vim.on_key()` approach to listen to typed keys. It will listen
to keys on the *left hand side* of mappings. The `on_key` callback is
cleared when detaching the last on-type formatting client. This feature
is disabled by default.

Co-authored-by: Maria José Solano <majosolano99@gmail.com>

test/functional/plugin/lsp/on_type_formatting_spec.lua

@@ -0,0 +1,174 @@
+local t = require('test.testutil')
+local n = require('test.functional.testnvim')()
+local t_lsp = require('test.functional.plugin.lsp.testutil')
+local retry = t.retry
+
+local eq = t.eq
+local dedent = t.dedent
+local exec_lua = n.exec_lua
+local insert = n.insert
+local feed = n.feed
+
+local clear_notrace = t_lsp.clear_notrace
+local create_server_definition = t_lsp.create_server_definition
+
+describe('vim.lsp.on_type_formatting', function()
+  local text = dedent([[
+  int main() {
+    int hi
+  }]])
+
+  before_each(function()
+    clear_notrace()
+
+    exec_lua(create_server_definition)
+    exec_lua(function()
+      _G.server = _G._create_server({
+        capabilities = {
+          documentOnTypeFormattingProvider = {
+            firstTriggerCharacter = '=',
+          },
+        },
+        handlers = {
+          ---@param params lsp.DocumentOnTypeFormattingParams
+          ---@param callback fun(err?: lsp.ResponseError, result?: lsp.TextEdit[])
+          ['textDocument/onTypeFormatting'] = function(_, params, callback)
+            callback(nil, {
+              {
+                newText = ';',
+                range = {
+                  start = params.position,
+                  ['end'] = params.position,
+                },
+              },
+            })
+          end,
+        },
+      })
+
+      _G.server_id = vim.lsp.start({
+        name = 'dummy',
+        cmd = _G.server.cmd,
+      })
+      vim.lsp.on_type_formatting.enable(true, { client_id = _G.server_id })
+    end)
+
+    insert(text)
+  end)
+
+  it('enables formatting on type', function()
+    exec_lua(function()
+      local win = vim.api.nvim_get_current_win()
+      vim.api.nvim_win_set_cursor(win, { 2, 0 })
+    end)
+    feed('A = 5')
+    retry(nil, 100, function()
+      eq(
+        {
+          'int main() {',
+          '  int hi = 5;',
+          '}',
+        },
+        exec_lua(function()
+          return vim.api.nvim_buf_get_lines(0, 0, -1, false)
+        end)
+      )
+    end)
+  end)
+
+  it('works with multiple clients', function()
+    exec_lua(function()
+      vim.lsp.on_type_formatting.enable(true)
+      _G.server2 = _G._create_server({
+        capabilities = {
+          documentOnTypeFormattingProvider = {
+            firstTriggerCharacter = '.',
+            moreTriggerCharacter = { '=' },
+          },
+        },
+        handlers = {
+          ---@param params lsp.DocumentOnTypeFormattingParams
+          ---@param callback fun(err?: lsp.ResponseError, result?: lsp.TextEdit[])
+          ['textDocument/onTypeFormatting'] = function(_, params, callback)
+            callback(nil, {
+              {
+                newText = ';',
+                range = {
+                  start = params.position,
+                  ['end'] = params.position,
+                },
+              },
+            })
+          end,
+        },
+      })
+
+      vim.lsp.start({
+        name = 'dummy2',
+        cmd = _G.server2.cmd,
+      })
+      local win = vim.api.nvim_get_current_win()
+      vim.api.nvim_win_set_cursor(win, { 2, 0 })
+    end)
+    feed('A =')
+    retry(nil, 100, function()
+      eq(
+        {
+          'int main() {',
+          '  int hi =;;',
+          '}',
+        },
+        exec_lua(function()
+          return vim.api.nvim_buf_get_lines(0, 0, -1, false)
+        end)
+      )
+    end)
+  end)
+
+  it('can be disabled', function()
+    exec_lua(function()
+      vim.lsp.on_type_formatting.enable(false, { client_id = _G.server_id })
+      local win = vim.api.nvim_get_current_win()
+      vim.api.nvim_win_set_cursor(win, { 2, 0 })
+    end)
+    feed('A = 5')
+    eq(
+      {
+        'int main() {',
+        '  int hi = 5',
+        '}',
+      },
+      exec_lua(function()
+        return vim.api.nvim_buf_get_lines(0, 0, -1, false)
+      end)
+    )
+  end)
+
+  it('attaches to new buffers', function()
+    exec_lua(function()
+      local buf = vim.api.nvim_create_buf(true, false)
+      vim.api.nvim_set_current_buf(buf)
+      vim.api.nvim_buf_set_lines(buf, 0, -1, false, {
+        'int main() {',
+        '  int hi',
+        '}',
+      })
+      local win = vim.api.nvim_get_current_win()
+      vim.api.nvim_win_set_cursor(win, { 2, 0 })
+      vim.lsp.buf_attach_client(buf, _G.server_id)
+    end)
+    feed('A = 5')
+    retry(nil, 100, function()
+      eq(
+        {
+          'int main() {',
+          '  int hi = 5;',
+          '}',
+        },
+        exec_lua(function()
+          return vim.api.nvim_buf_get_lines(0, 0, -1, false)
+        end)
+      )
+    end)
+  end)
+end)