Prompt library updates (#11988)

Restructure prompts & the prompt library.

- Prompts are now written in markdown
- The prompt manager has a picker and editable prompts
- Saving isn't wired up yet
- This also removes the "Insert active prompt" button as this concept doesn't exist anymore, and will be replaced with slash commands.

I didn't staff flag this, but if you do play around with it expect it to still be pretty rough.

Release Notes:

- N/A

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Co-authored-by: Nathan Sobo <1789+nathansobo@users.noreply.github.com>
Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/assistant/Cargo.toml

@@ -50,6 +50,8 @@ ui.workspace = true
 util.workspace = true
 uuid.workspace = true
 workspace.workspace = true
+picker.workspace = true
+gray_matter = "0.2.7"
 
 [dev-dependencies]
 ctor.workspace = true