This PR implements a UI for the side-by-side diff, using blocks to align
the two sides and adding a coherent `SplitEditorElement`.

Release Notes:

- N/A

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Co-authored-by: cameron <cameron.studdstreet@gmail.com>
Co-authored-by: Anthony Eid <hello@anthonyeid.me>
Co-authored-by: Zed Zippy <234243425+zed-zippy[bot]@users.noreply.github.com>