feat(lsp): add vim.lsp.config and vim.lsp.enable

Design goals/requirements:
- Default configuration of a server can be distributed across multiple sources.
  - And via RTP discovery.
- Default configuration can be specified for all servers.
- Configuration _can_ be project specific.

Solution:

- Two new API's:
  - `vim.lsp.config(name, cfg)`:
    - Used to define default configurations for servers of name.
    - Can be used like a table or called as a function.
    - Use `vim.lsp.confg('*', cfg)` to specify default config for all
      servers.
  - `vim.lsp.enable(name)`
    - Used to enable servers of name. Uses configuration defined
    via `vim.lsp.config()`.

test/functional/plugin/lsp_spec.lua

@@ -6098,15 +6098,6 @@ describe('LSP', function()
       end
 
       eq(is_os('mac') or is_os('win'), check_registered(nil)) -- start{_client}() defaults to make_client_capabilities().
-      eq(false, check_registered(vim.empty_dict()))
-      eq(
-        false,
-        check_registered({
-          workspace = {
-            ignoreMe = true,
-          },
-        })
-      )
       eq(
         false,
         check_registered({
@@ -6129,4 +6120,88 @@ describe('LSP', function()
       )
     end)
   end)
+
+  describe('vim.lsp.config() and vim.lsp.enable()', function()
+    it('can merge settings from "*"', function()
+      eq(
+        {
+          name = 'foo',
+          cmd = { 'foo' },
+          root_markers = { '.git' },
+        },
+        exec_lua(function()
+          vim.lsp.config('*', { root_markers = { '.git' } })
+          vim.lsp.config('foo', { cmd = { 'foo' } })
+
+          return vim.lsp._resolve_config('foo')
+        end)
+      )
+    end)
+
+    it('sets up an autocmd', function()
+      eq(
+        1,
+        exec_lua(function()
+          vim.lsp.config('foo', {
+            cmd = { 'foo' },
+            root_markers = { '.foorc' },
+          })
+          vim.lsp.enable('foo')
+          return #vim.api.nvim_get_autocmds({
+            group = 'nvim.lsp.enable',
+            event = 'FileType',
+          })
+        end)
+      )
+    end)
+
+    it('attaches to buffers', function()
+      exec_lua(create_server_definition)
+
+      local tmp1 = t.tmpname(true)
+      local tmp2 = t.tmpname(true)
+
+      exec_lua(function()
+        local server = _G._create_server({
+          handlers = {
+            initialize = function(_, _, callback)
+              callback(nil, { capabilities = {} })
+            end,
+          },
+        })
+
+        vim.lsp.config('foo', {
+          cmd = server.cmd,
+          filetypes = { 'foo' },
+          root_markers = { '.foorc' },
+        })
+
+        vim.lsp.config('bar', {
+          cmd = server.cmd,
+          filetypes = { 'bar' },
+          root_markers = { '.foorc' },
+        })
+
+        vim.lsp.enable('foo')
+        vim.lsp.enable('bar')
+
+        vim.cmd.edit(tmp1)
+        vim.bo.filetype = 'foo'
+        _G.foo_buf = vim.api.nvim_get_current_buf()
+
+        vim.cmd.edit(tmp2)
+        vim.bo.filetype = 'bar'
+        _G.bar_buf = vim.api.nvim_get_current_buf()
+      end)
+
+      eq(
+        { 1, 'foo', 1, 'bar' },
+        exec_lua(function()
+          local foos = vim.lsp.get_clients({ bufnr = assert(_G.foo_buf) })
+          local bars = vim.lsp.get_clients({ bufnr = assert(_G.bar_buf) })
+          return { #foos, foos[1].name, #bars, bars[1].name }
+        end)
+      )
+    end)
+  end)
 end)