Extract some of the improvements from to the unified diff prompt from
https://github.com/zed-industries/zed/pull/42171 and adds some other
about how context work to improve the reliability of predictions.

We also now strip the `<|user_cursor|>` marker if it appears in the
output rather than failing.

Release Notes:

- N/A

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Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>