Extract Protocol Buffers support into an extension (#18704)

This PR extracts the Protocol Buffers support into an extension.

Release Notes:

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

crates/extensions_ui/src/extension_suggest.rs

@@ -54,6 +54,7 @@ const SUGGESTIONS_BY_EXTENSION_ID: &[(&str, &[&str])] = &[
     ("ocaml", &["ml", "mli"]),
     ("php", &["php"]),
     ("prisma", &["prisma"]),
+    ("proto", &["proto"]),
     ("purescript", &["purs"]),
     ("r", &["r", "R"]),
     ("racket", &["rkt"]),