lsp: Support Goto Declaration (#15785)

Adds support for [Goto
Declaration](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_declaration)
LSP command.

I am particularly interested in [this for Rust
projects](https://rust-analyzer.github.io/manual.html#go-to-declaration),
to be able to navigate to the place where a trait method is declared,
coming from a trait method implementation.

I noticed this was something I could do in VSCode before, but was
somehow missing is Zed. Thanks to the already existing infrastructure
for Goto Definition, I just followed and copy-paste-adapted it for Goto
Declaration.

As a bonus, I added `ctrl-F12` and `alt-ctrl-F12` as default macOS
keybindings for `GoToDeclaration` and `GoToDeclarationSplit`,
respectively. They are not keybindings from another editor, but I
figured they made sense to be grouped along with the other *F12
commands.

### Release Notes:

- Added "Go to declaration" editor action.
- vim: Breaking change to keybindings after introduction of the `Go to
declaration` editor action. The new keybindings are the following (and
can be found [here](https://zed.dev/docs/vim), alongside the other key
bindings):
  - `g d` - Go to definition
  - `g D` - Go to declaration
  - `g y` - Go to type definition
  - `g I` - Go to implementation




https://github.com/user-attachments/assets/ee5c10a8-94f0-4e50-afbb-6f71db540c1b

---------

Co-authored-by: Thorsten Ball <mrnugget@gmail.com>

crates/proto/proto/zed.proto

@@ -48,6 +48,8 @@ message Envelope {
 
         GetDefinition get_definition = 32;
         GetDefinitionResponse get_definition_response = 33;
+        GetDeclaration get_declaration = 237;
+        GetDeclarationResponse get_declaration_response = 238; // current max
         GetTypeDefinition get_type_definition = 34;
         GetTypeDefinitionResponse get_type_definition_response = 35;
 
@@ -696,12 +698,23 @@ message GetDefinition {
      uint64 buffer_id = 2;
      Anchor position = 3;
      repeated VectorClockEntry version = 4;
- }
+}
 
 message GetDefinitionResponse {
     repeated LocationLink links = 1;
 }
 
+message GetDeclaration {
+     uint64 project_id = 1;
+     uint64 buffer_id = 2;
+     Anchor position = 3;
+     repeated VectorClockEntry version = 4;
+}
+
+message GetDeclarationResponse {
+    repeated LocationLink links = 1;
+}
+
 message GetTypeDefinition {
      uint64 project_id = 1;
      uint64 buffer_id = 2;

crates/proto/src/proto.rs

@@ -239,6 +239,8 @@ messages!(
     (GetCompletionsResponse, Background),
     (GetDefinition, Background),
     (GetDefinitionResponse, Background),
+    (GetDeclaration, Background),
+    (GetDeclarationResponse, Background),
     (GetDocumentHighlights, Background),
     (GetDocumentHighlightsResponse, Background),
     (GetHover, Background),
@@ -437,6 +439,7 @@ request_messages!(
     (GetCodeActions, GetCodeActionsResponse),
     (GetCompletions, GetCompletionsResponse),
     (GetDefinition, GetDefinitionResponse),
+    (GetDeclaration, GetDeclarationResponse),
     (GetImplementation, GetImplementationResponse),
     (GetDocumentHighlights, GetDocumentHighlightsResponse),
     (GetHover, GetHoverResponse),
@@ -551,6 +554,7 @@ entity_messages!(
     GetCodeActions,
     GetCompletions,
     GetDefinition,
+    GetDeclaration,
     GetImplementation,
     GetDocumentHighlights,
     GetHover,