Add editor::OpenUrl action and bind to `gx` in Vim mode (#7972)

This adds one feature I've been missing a lot in Vim mode: `gx` to open
the URL under the cursor.

Technically, in Vim, `gx` opens more "paths", not just URLs, but I think
this is a good start.

Release Notes:

- Added `gx` to Vim mode to open the URL under the cursor.

Demo:


https://github.com/zed-industries/zed/assets/1185253/6a19490d-b61d-40b7-93e8-4819599f6977