Rework context insertion UX (#12360)

- Confirming a completion now runs the command immediately
- Hitting `enter` on a line with a command now runs it
- The output of commands gets folded away and replaced with a custom
placeholder
- Eliminated ambient context

<img width="1588" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/b1927a45-52d6-4634-acc9-2ee539c1d89a">

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>

extensions/gleam/extension.toml

@@ -17,3 +17,4 @@ commit = "8432ffe32ccd360534837256747beb5b1c82fca1"
 [slash_commands.gleam-project]
 description = "Returns information about the current Gleam project."
 requires_argument = false
+tooltip_text = "Insert Gleam project data"