Allow extensions to define more of the methods in the LspAdapter trait (#9554)

Our goal is to extract Svelte support into an extension, since we've
seen problems with the Tree-sitter Svelte parser crashing due to bugs in
the external scanner. In order to do this, we need a couple more
capabilities in LSP extensions:

* [x] `initialization_options` - programmatically controlling the JSON
initialization params sent to the language server
* [x] `prettier_plugins` - statically specifying a list of prettier
plugins that apply for a given language.
* [x] `npm_install_package`

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/languages/src/php/config.toml

@@ -15,3 +15,4 @@ collapsed_placeholder = "/* ... */"
 word_characters = ["$"]
 scope_opt_in_language_servers = ["tailwindcss-language-server"]
 prettier_parser_name = "php"
+prettier_plugins = ["@prettier/plugin-php"]