Add support for using a language server with multiple languages (#10293)

This PR updates the `extension.toml` to allow specifying multiple
languages for a language server to work with.

The `languages` field takes precedence over `language`. In the future
the `language` field will be removed.

As part of this, the Emmet extension has been extended with support for
PHP and ERB.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>

extensions/emmet/extension.toml

@@ -9,3 +9,4 @@ repository = "https://github.com/zed-industries/zed"
 [language_servers.emmet-language-server]
 name = "Emmet Language Server"
 language = "HTML"
+languages = ["HTML", "PHP", "ERB"]