Reuse prompt editor across buffer and terminal assist (#22188)

Builds on https://github.com/zed-industries/zed/pull/22160 and extracts
the rest of `PromptEditor` so it can be shared across terminal and
inline assistants. This will help avoid the UI drifting as we have
already observed.

Note: This is mostly a mechanical refactor. I imagine some things could
be factored in a better way by someone with more context, but I think
this is a good start.

Release Notes:

- N/A

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Co-authored-by: Richard Feldman <oss@rtfeldman.com>