feat: filetype.lua (#16600)

Adds a new vim.filetype module that provides support for filetype detection in
Lua.

test/functional/lua/filetype_spec.lua

@@ -0,0 +1,100 @@
+local helpers = require('test.functional.helpers')(after_each)
+local exec_lua = helpers.exec_lua
+local eq = helpers.eq
+local clear = helpers.clear
+
+describe('vim.filetype', function()
+  before_each(function()
+    clear()
+
+    exec_lua [[
+      local bufnr = vim.api.nvim_create_buf(true, false)
+      vim.api.nvim_set_current_buf(bufnr)
+
+    ]]
+  end)
+
+  it('works with extensions', function()
+    eq('radicalscript', exec_lua [[
+      vim.filetype.add({
+        extension = {
+          rs = 'radicalscript',
+        },
+      })
+      vim.api.nvim_buf_set_name(0, '/home/user/src/main.rs')
+      vim.filetype.match(0)
+      return vim.bo.filetype
+    ]])
+  end)
+
+  it('prioritizes filenames over extensions', function()
+    eq('somethingelse', exec_lua [[
+      vim.filetype.add({
+        extension = {
+          rs = 'radicalscript',
+        },
+        filename = {
+          ['main.rs'] = 'somethingelse',
+        },
+      })
+      vim.api.nvim_buf_set_name(0, '/home/usr/src/main.rs')
+      vim.filetype.match(0)
+      return vim.bo.filetype
+    ]])
+  end)
+
+  it('works with filenames', function()
+    eq('nim', exec_lua [[
+      vim.filetype.add({
+        filename = {
+          ['s_O_m_e_F_i_l_e'] = 'nim',
+        },
+      })
+      vim.api.nvim_buf_set_name(0, '/home/user/src/s_O_m_e_F_i_l_e')
+      vim.filetype.match(0)
+      return vim.bo.filetype
+    ]])
+
+    eq('dosini', exec_lua [[
+      vim.filetype.add({
+        filename = {
+          ['config'] = 'toml',
+          ['~/.config/fun/config'] = 'dosini',
+        },
+      })
+      vim.api.nvim_buf_set_name(0, '~/.config/fun/config')
+      vim.filetype.match(0)
+      return vim.bo.filetype
+    ]])
+  end)
+
+  it('works with patterns', function()
+    eq('markdown', exec_lua [[
+      vim.filetype.add({
+        pattern = {
+          ['~/blog/.*%.txt'] = 'markdown',
+        }
+      })
+      vim.api.nvim_buf_set_name(0, '~/blog/why_neovim_is_awesome.txt')
+      vim.filetype.match(0)
+      return vim.bo.filetype
+    ]])
+  end)
+
+  it('works with functions', function()
+    eq('foss', exec_lua [[
+      vim.filetype.add({
+        pattern = {
+          ["relevant_to_(%a+)"] = function(path, bufnr, capture)
+            if capture == "me" then
+              return "foss"
+            end
+          end,
+        }
+      })
+      vim.api.nvim_buf_set_name(0, 'relevant_to_me')
+      vim.filetype.match(0)
+      return vim.bo.filetype
+    ]])
+  end)
+end)