Extract PHP support into an extension (#9966)

This PR extracts PHP support into an extension and removes the built-in
PHP support from Zed.

There's a small workaround necessary in order for us to provide the
`language_ids` on the `LspAdapter` that are needed for the language
server to run properly. Eventually we'll want to build this into the
extension API, but for now we're just hard-coding it on the host side.

Release Notes:

- Removed built-in support for PHP, in favor of making it available as
an extension. The PHP extension will be suggested for download when you
open a `.php` file.

extensions/php/languages/php/highlights.scm

@@ -0,0 +1,124 @@
+(php_tag) @tag
+"?>" @tag
+
+; Types
+
+(primitive_type) @type.builtin
+(cast_type) @type.builtin
+(named_type (name) @type) @type
+(named_type (qualified_name) @type) @type
+
+; Functions
+
+(array_creation_expression "array" @function.builtin)
+(list_literal "list" @function.builtin)
+
+(method_declaration
+  name: (name) @function.method)
+
+(function_call_expression
+  function: [(qualified_name (name)) (name)] @function)
+
+(scoped_call_expression
+  name: (name) @function)
+
+(member_call_expression
+  name: (name) @function.method)
+
+(function_definition
+  name: (name) @function)
+
+; Member
+
+(property_element
+  (variable_name) @property)
+
+(member_access_expression
+  name: (variable_name (name)) @property)
+(member_access_expression
+  name: (name) @property)
+
+; Variables
+
+(relative_scope) @variable.builtin
+
+((name) @constant
+ (#match? @constant "^_?[A-Z][A-Z\\d_]+$"))
+((name) @constant.builtin
+ (#match? @constant.builtin "^__[A-Z][A-Z\d_]+__$"))
+
+((name) @constructor
+ (#match? @constructor "^[A-Z]"))
+
+((name) @variable.builtin
+ (#eq? @variable.builtin "this"))
+
+(variable_name) @variable
+
+; Basic tokens
+[
+  (string)
+  (string_value)
+  (encapsed_string)
+  (heredoc)
+  (heredoc_body)
+  (nowdoc_body)
+] @string
+(boolean) @constant.builtin
+(null) @constant.builtin
+(integer) @number
+(float) @number
+(comment) @comment
+
+"$" @operator
+
+; Keywords
+
+"abstract" @keyword
+"as" @keyword
+"break" @keyword
+"case" @keyword
+"catch" @keyword
+"class" @keyword
+"const" @keyword
+"continue" @keyword
+"declare" @keyword
+"default" @keyword
+"do" @keyword
+"echo" @keyword
+"else" @keyword
+"elseif" @keyword
+"enum" @keyword
+"enddeclare" @keyword
+"endforeach" @keyword
+"endif" @keyword
+"endswitch" @keyword
+"endwhile" @keyword
+"extends" @keyword
+"final" @keyword
+"readonly" @keyword
+"finally" @keyword
+"foreach" @keyword
+"function" @keyword
+"global" @keyword
+"if" @keyword
+"implements" @keyword
+"include_once" @keyword
+"include" @keyword
+"insteadof" @keyword
+"interface" @keyword
+"namespace" @keyword
+"new" @keyword
+"private" @keyword
+"protected" @keyword
+"public" @keyword
+"require_once" @keyword
+"require" @keyword
+"return" @keyword
+"static" @keyword
+"switch" @keyword
+"throw" @keyword
+"trait" @keyword
+"try" @keyword
+"use" @keyword
+"while" @keyword