lsp: get_language_id (#14092)

* Allow specifying a languageId for a lsp

For some languages the filetype might not match the languageId the
language server accepts. In these cases the config for the language
server can contain a function which gets the current buffer and filetype
and returns a languageId. When it isn't provided the filetype is used
instead.

Example:
```lua
require'lspconfig'.sourcekit.setup{
    get_language_id = function(bufnr, ft)
        return 'swift'
    end;
}
```

Closes #13093

* lsp: Change to get_language_id

Co-authored-by: Jan Dammshäuser <mail@jandamm.de>

1 files changed, 2 insertions, 0 deletions

diff --git a/runtime/doc/lsp.txt b/runtime/doc/lsp.txt
index 24f9dd7a69..6d7c3f26ef 100644
--- a/runtime/doc/lsp.txt
+++ b/runtime/doc/lsp.txt
@@ -799,6 +799,8 @@ start_client({config})                                *vim.lsp.start_client()*
                                        See `initialize` in the LSP spec.
                     {name}             (string, default=client-id) Name in log
                                        messages.
+                    {get_language_id}  function(bufnr, filetype) -> language
+                                       ID as string. Defaults to the filetype.
                     {offset_encoding}  (default="utf-16") One of "utf-8",
                                        "utf-16", or "utf-32" which is the
                                        encoding that the LSP server expects.