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>

docs/src/key-bindings.md

@@ -218,6 +218,8 @@ See the [tasks documentation](/docs/tasks#custom-keybindings-for-tasks) for more
 | Format                           | Editor     | `⌘ + Shift + I`                 |
 | Go to definition                 | Editor     | `F12`                           |
 | Go to definition split           | Editor     | `Alt + F12`                     |
+| Go to declaration                | Editor     | `Ctrl + F12`                    |
+| Go to declaration split          | Editor     | `Alt + Ctrl + F12`              |
 | Go to diagnostic                 | Editor     | `F8`                            |
 | Go to implementation             | Editor     | `Shift + F12`                   |
 | Go to prev diagnostic            | Editor     | `Shift + F8`                    |

docs/src/vim.md

@@ -17,8 +17,10 @@ Vim mode has several "core Zed" key bindings, that will help you make the most o
 ```
 # Language server
 g d     Go to definition
-g D     Go to type definition
-g cmd-d Go to implementation
+g D     Go to declaration
+g y     Go to type definition
+g I     Go to implementation
+
 c d     Rename (change definition)
 g A     Go to All references to the current word