Add a slash command for automatically retrieving relevant context (#17972)

* [x] put this slash command behind a feature flag until we release
embedding access to the general population
* [x] choose a name for this slash command and name the rust module to
match

Release Notes:

- N/A

---------

Co-authored-by: Jason <jason@zed.dev>
Co-authored-by: Richard <richard@zed.dev>
Co-authored-by: Jason Mancuso <7891333+jvmncs@users.noreply.github.com>
Co-authored-by: Richard Feldman <oss@rtfeldman.com>

assets/prompts/project_slash_command.hbs

@@ -0,0 +1,8 @@
+A software developer is asking a question about their project. The source files in their project have been indexed into a database of semantic text embeddings.
+Your task is to generate a list of 4 diverse search queries that can be run on this embedding database, in order to retrieve a list of code snippets
+that are relevant to the developer's question. Redundant search queries will be heavily penalized, so only include another query if it's sufficiently
+distinct from previous ones.
+
+Here is the question that's been asked, together with context that the developer has added manually:
+
+{{{context_buffer}}}