Extract slash commands to their own crate (#23261)

This PR extracts the slash command definitions out of the `assistant`
crate and into their own `assistant_slash_commands` crate.

Release Notes:

- N/A

crates/assistant_slash_commands/src/prompt_before_summary.txt

@@ -0,0 +1,31 @@
+I'm going to give you a prompt. I don't want you to respond
+to the prompt itself. I want you to figure out which of the following
+actions on my project, if any, would help you answer the prompt.
+
+Here are the actions:
+
+## file
+
+This action's parameter is a file path to one of the files
+in the project. If you ask for this action, I will tell you
+the full contents of the file, so you  can learn all the
+details of the file.
+
+## search
+
+This action's parameter is a string to do a semantic search for
+across the files in the project. (You will have a JSON summary
+of all the files in the project.) It will tell you which files this string
+(or similar strings; it is a semantic search) appear in,
+as well as some context of the lines surrounding each result.
+It's very important that you only use this action when you think
+that searching across the specific files in this project for the query
+in question will be useful. For example, don't use this command to search
+for queries you might put into a general Web search engine, because those
+will be too general to give useful results in this project-specific search.
+
+---
+
+That was the end of the list of actions.
+
+Here is a JSON summary of each of the files in my project: