debugger: Rework language association with the debuggers (#29945)

- Languages now define their preferred debuggers in `config.toml`.
- `LanguageRegistry` now exposes language config even for languages that
are not yet loaded. This necessitated extension registry changes (we now
deserialize config.toml of all language entries when loading new
extension index), but it should be backwards compatible with the old
format. /cc @maxdeviant

Release Notes:

- N/A

---------

Co-authored-by: Anthony Eid <hello@anthonyeid.me>
Co-authored-by: Remco Smits <djsmits12@gmail.com>
Co-authored-by: Anthony <anthony@zed.dev>

crates/language/src/language_settings.rs

@@ -153,6 +153,8 @@ pub struct LanguageSettings {
     pub show_completion_documentation: bool,
     /// Completion settings for this language.
     pub completions: CompletionSettings,
+    /// Preferred debuggers for this language.
+    pub debuggers: Vec<String>,
 }
 
 impl LanguageSettings {
@@ -551,6 +553,10 @@ pub struct LanguageSettingsContent {
     pub show_completion_documentation: Option<bool>,
     /// Controls how completions are processed for this language.
     pub completions: Option<CompletionSettings>,
+    /// Preferred debuggers for this language.
+    ///
+    /// Default: []
+    pub debuggers: Option<Vec<String>>,
 }
 
 /// The behavior of `editor::Rewrap`.