Adjust editor: open excerpts split key binding

[JosephTLyons]

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.

[zed-industries-bot]

	Warnings
⚠️	Adjust `editor: open excerpts split` key binding ^ Write PR titles using sentence case.

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