helix: Add `vim::HelixGotoLine` action and bind it to `G` (#61581)

# Objective

Make `G` keybinding in Helix mode work like in Helix and not like in
Vim.

Fixes https://github.com/zed-industries/zed/issues/61580

## Solution

Helix has two ways to jump to a line by line number.

One is the `goto_file_start` command (bound to `gg`) that optionally
takes a count to go to that line instead of the start of the file. Zed
already supports it as `vim::StartOfDocument`.

The other is the dedicated `goto_line` command (bound to `G`) that only
does that and nothing else. Zed did not have it.

What's worse, the default `"shift-g": "vim::EndOfDocument"` binding
leaked from Vim keymap into Helix keymap, which previously made
`<count>G` accidentally work in Helix mode for the wrong reason, until
https://github.com/zed-industries/zed/pull/59449 fixed the behavior of
`vim::StartOfDocument` and `vim::EndOfDocument` actions to match Helix
exactly. This broke `<count>G` and exposed that `G` was bound to the
wrong action in Helix mode, and the correct one didn't exist.

This PR fixes that in the following way:
- adds new`vim::HelixGotoLine` action
- binds it to `shift-g` in `helix_normal` and `helix_select` modes in
the default Vim keymap

## Testing

- Unit tests
- Manual testing

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

---

Release Notes:

- Fixed the behavior of `G` binding in Helix mode and added new
`vim::HelixGotoLine` action

Signed-off-by: Oleksii Orlenko <alex@aqrln.net>