Allow prompt templates to be overridden in the zed configuration directory (#15887)

I need this to refine our prompts on the fly as I work.

Release Notes:

- Templates for prompts driving inline transformation in editors and the
terminal can now be overridden in the `~/.config/zed/prompts/templates`
directory. This is an advanced feature, and prevents you from getting
upstream changes. It's intended for use by Zed developers.