fix: support compound file-types

Vim supports compound filetypes (see |'filetype'|):

> When a dot appears in the value then this separates two filetype names.
> This works both for filetype plugins and for syntax files. More than one dot may appear.

nvim-lspconfig currently always compares the full 'filetype' option
against the configured filetypes. So buffers set to multiple filetypes
will not match at all.

Fix utility functions to match against each individual filetype

lua/lspconfig/server_configurations/elmls.lua

@@ -10,7 +10,7 @@ return {
     filetypes = { 'elm' },
     root_dir = function(fname)
       local filetype = api.nvim_buf_get_option(0, 'filetype')
-      if filetype == 'elm' or (filetype == 'json' and fname:match 'elm%.json$') then
+      if util.ft_matches(filetype, 'elm') or (util.ft_matches(filetype, 'json') and fname:match 'elm%.json$') then
         return elm_root_pattern(fname)
       end
     end,

lua/lspconfig/util.lua

@@ -22,6 +22,21 @@ M.default_config = {
 -- global on_setup hook
 M.on_setup = nil
 
+---@param filetype string the filetype to check (can be a compound, dot-separated filetype; see |'filetype'|)
+---@param expected string|string[] the filetype(s) to match against
+---@return boolean
+function M.ft_matches(filetype, expected)
+  expected = type(expected) == 'table' and expected or { expected }
+  for ft in filetype:gmatch '([^.]+)' do
+    for _, expected_ft in ipairs(expected) do
+      if ft == expected_ft then
+        return true
+      end
+    end
+  end
+  return false
+end
+
 function M.bufname_valid(bufname)
   if bufname:match '^/' or bufname:match '^[a-zA-Z]:' or bufname:match '^zipfile://' or bufname:match '^tarfile:' then
     return true
@@ -348,10 +363,8 @@ function M.get_active_clients_list_by_ft(filetype)
   local clients_list = {}
   for _, client in pairs(clients) do
     local filetypes = client.config.filetypes or {}
-    for _, ft in pairs(filetypes) do
-      if ft == filetype then
-        table.insert(clients_list, client.name)
-      end
+    if M.ft_matches(filetype, filetypes) then
+      table.insert(clients_list, client.name)
     end
   end
   return clients_list
@@ -364,10 +377,8 @@ function M.get_other_matching_providers(filetype)
   for _, config in pairs(configs) do
     if not vim.tbl_contains(active_clients_list, config.name) then
       local filetypes = config.filetypes or {}
-      for _, ft in pairs(filetypes) do
-        if ft == filetype then
-          table.insert(other_matching_configs, config)
-        end
+      if M.ft_matches(filetype, filetypes) then
+        table.insert(other_matching_configs, config)
       end
     end
   end
@@ -379,10 +390,8 @@ function M.get_config_by_ft(filetype)
   local matching_configs = {}
   for _, config in pairs(configs) do
     local filetypes = config.filetypes or {}
-    for _, ft in pairs(filetypes) do
-      if ft == filetype then
-        table.insert(matching_configs, config)
-      end
+    if M.ft_matches(filetype, filetypes) then
+      table.insert(matching_configs, config)
     end
   end
   return matching_configs