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.

crates/assets/src/assets.rs

@@ -11,6 +11,7 @@ use rust_embed::RustEmbed;
 #[include = "themes/**/*"]
 #[exclude = "themes/src/*"]
 #[include = "sounds/**/*"]
+#[include = "prompts/**/*"]
 #[include = "*.md"]
 #[exclude = "*.DS_Store"]
 pub struct Assets;