Following #7665, I've added a keymap to quickly hide and show gutter
line numbers.

`ctrl-l` and `cmd-l` were taken, so I've bound it to `cmd-;`. 



https://github.com/zed-industries/zed/assets/138762/365d2a7c-b775-4486-8389-edafe59b2a87

Release notes:

- Added `editor: toggle line numbers` command and default keybindings
(`cmd-;` on macOS).

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Co-authored-by: Thorsten Ball <mrnugget@gmail.com>