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>

assets/keymaps/default-macos.json

@@ -127,7 +127,9 @@
       "cmd-'": "editor::ToggleHunkDiff",
       "cmd-\"": "editor::ExpandAllHunkDiffs",
       "cmd-alt-g b": "editor::ToggleGitBlame",
-      "cmd-i": "editor::ShowSignatureHelp"
+      "cmd-i": "editor::ShowSignatureHelp",
+      "ctrl-f12": "editor::GoToDeclaration",
+      "alt-ctrl-f12": "editor::GoToDeclarationSplit"
     }
   },
   {

assets/keymaps/vim.json

@@ -89,8 +89,9 @@
       "g t": "pane::ActivateNextItem",
       "g shift-t": "pane::ActivatePrevItem",
       "g d": "editor::GoToDefinition",
-      "g shift-d": "editor::GoToTypeDefinition",
-      "g cmd-d": "editor::GoToImplementation",
+      "g shift-d": "editor::GoToDeclaration",
+      "g y": "editor::GoToTypeDefinition",
+      "g shift-i": "editor::GoToImplementation",
       "g x": "editor::OpenUrl",
       "g n": "vim::SelectNextMatch",
       "g shift-n": "vim::SelectPreviousMatch",