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>

crates/vim/src/state.rs

@@ -68,6 +68,8 @@ pub enum Operator {
     Object { around: bool },
     FindForward { before: bool },
     FindBackward { after: bool },
+    Sneak { first_char: Option<char> },
+    SneakBackward { first_char: Option<char> },
     AddSurrounds { target: Option<SurroundsType> },
     ChangeSurrounds { target: Option<Object> },
     DeleteSurrounds,
@@ -460,6 +462,8 @@ impl Operator {
             Operator::Literal { .. } => "^V",
             Operator::FindForward { before: false } => "f",
             Operator::FindForward { before: true } => "t",
+            Operator::Sneak { .. } => "s",
+            Operator::SneakBackward { .. } => "S",
             Operator::FindBackward { after: false } => "F",
             Operator::FindBackward { after: true } => "T",
             Operator::AddSurrounds { .. } => "ys",
@@ -502,6 +506,8 @@ impl Operator {
             | Operator::Mark
             | Operator::Jump { .. }
             | Operator::FindBackward { .. }
+            | Operator::Sneak { .. }
+            | Operator::SneakBackward { .. }
             | Operator::Register
             | Operator::RecordRegister
             | Operator::ReplayRegister