assistant: Make scripting a first-class concept instead of a tool (#26338)

This PR makes refactors the scripting functionality to be a first-class
concept of the assistant instead of a generic tool, which will allow us
to build a more customized experience.

- The tool prompt has been slightly tweaked and is now included as a
system message in all conversations. I'm getting decent results, but now
that it isn't in the tools framework, it will probably require more
refining.

- The model will now include an `<eval ...>` tag at the end of the
message with the script. We parse this tag incrementally as it streams
in so that we can indicate that we are generating a script before we see
the closing `</eval>` tag. Later, this will help us interpret the script
as it arrives also.

- Threads now hold a `ScriptSession` entity which manages the state of
all scripts (from parsing to exited) in a centralized way, and will
later collect all script operations so they can be displayed in the UI.

- `script_tool` has been renamed to `assistant_scripting` 

- Script source now opens in a regular read-only buffer  

Note: We still need to handle persistence properly

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/zed/Cargo.toml

@@ -98,7 +98,6 @@ remote.workspace = true
 repl.workspace = true
 reqwest_client.workspace = true
 rope.workspace = true
-scripting_tool.workspace = true
 search.workspace = true
 serde.workspace = true
 serde_json.workspace = true

crates/zed/src/main.rs

@@ -476,7 +476,6 @@ fn main() {
             cx,
         );
         assistant_tools::init(cx);
-        scripting_tool::init(cx);
         repl::init(app_state.fs.clone(), cx);
         extension_host::init(
             extension_host_proxy,