Adjust `editor: open excerpts split` key binding (#23646)

The pattern in Zed and in other editors is to use `cmd` to modify some
file-opening action to open it in a split rather than in the current
pane.

- In the project pane, a `click` opens a file, and a `cmd-click` opens
it in a split
- In the file finder, `enter` opens the file, and a `cmd-enter` opens it
in a split

It makes sense to me that if `alt-enter` opens a file from the excerpt,
that `cmd-alt-enter` opens it in a split, following the pattern above.

Note: I'm not auto-merging this, as others might disagree.
Note: I didn't touch the Vim binding.

Release Notes:

- Breaking Change: Changed `editor: open excerpts split` key binding to
`cmd-alt-enter` on macOS and `ctrl-alt-enter` on Linux.