Move plugins to feature-gate (#2599)

This PR:
- Reduces our dependency count for a release build by 20%
- Reduces our release compile time by about 30s (out of ~5 minutes)

crates/zed/src/languages.rs

@@ -10,6 +10,7 @@ mod elixir;
 mod go;
 mod html;
 mod json;
+#[cfg(feature = "plugin_runtime")]
 mod language_plugin;
 mod lua;
 mod python;