vim: Add sneak motion (#22793)

A (re)continuation of https://github.com/zed-industries/zed/pull/21067. 

This takes the original implementation in
https://github.com/zed-industries/zed/pull/15572 and adds the test in
https://github.com/zed-industries/zed/pull/21067. Then, as requested in
https://github.com/zed-industries/zed/pull/21067#issuecomment-2515469185,
it documents how to map a keybinding instead of having a setting.

Closes #13858

Release Notes:

- Added support for the popular
[vim_sneak](https://github.com/justinmk/vim-sneak) plugin. This is
disabled by default and can be enabled by binding a key to the `Sneak`
and `SneakBackward` operators.

Reference:
https://github.com/justinmk/vim-sneak

---------

Co-authored-by: Kajetan Puchalski <kajetan.puchalski@tuta.io>
Co-authored-by: Aidan Grant <mraidangrant@gmail.com>
Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

docs/src/vim.md

@@ -408,6 +408,20 @@ Vim mode comes with shortcuts to surround the selection in normal mode (`ys`), b
 }
 ```
 
+The [Sneak motion](https://github.com/justinmk/vim-sneak) feature allows for quick navigation to any two-character sequence in your text. You can enable it by adding the following keybindings to your keymap. By default, the `s` key is mapped to `vim::Substitute`. Adding these bindings will override that behavior, so ensure this change aligns with your workflow preferences.
+
+```json
+[
+  {
+    "context": "vim_mode == normal || vim_mode == visual",
+    "bindings": {
+      "s": ["vim::PushOperator", { "Sneak": {} }],
+      "S": ["vim::PushOperator", { "SneakBackward": {} }]
+    }
+  }
+]
+```
+
 ### Restoring common text editing keybindings
 
 If you're using vim mode on Linux or Windows, you may find it overrides keybindings you can't live without: `ctrl+v` to copy, `ctrl+f` to search, etc. You can restore them by copying this data into your keymap: