File context for assistant panel (#9712)

Introducing the Active File Context portion of #9705. When someone is in
the assistant panel it now includes the active file as a system message
on send while showing them a nice little display in the lower right:


![image](https://github.com/zed-industries/zed/assets/836375/9abc56e0-e8f2-45ee-9e7e-b83b28b483ea)

For this iteration, I'd love to see the following before we land this:

* [x] Toggle-able context - user should be able to disable sending this
context
* [x] Show nothing if there is no context coming in
* [x] Update token count as we change items
* [x] Listen for a more finely scoped event for when the active item
changes
* [x] Create a global for pulling a file icon based on a path. Zed's
main way to do this is nested within project panel's `FileAssociation`s.
* [x] Get the code fence name for a Language for the system prompt
* [x] Update the token count when the buffer content changes

I'm seeing this PR as the foundation for providing other kinds of
context -- diagnostic summaries, failing tests, additional files, etc.

Release Notes:

- Added file context to assistant chat panel
([#9705](https://github.com/zed-industries/zed/issues/9705)).

<img width="1558" alt="image"
src="https://github.com/zed-industries/zed/assets/836375/86eb7e50-3e28-4754-9c3f-895be588616d">

---------

Co-authored-by: Conrad Irwin <conrad@zed.dev>
Co-authored-by: Nathan <nathan@zed.dev>
Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Mikayla Maki <mikayla@zed.dev>

extensions/csharp/languages/csharp/config.toml

@@ -1,4 +1,5 @@
 name = "CSharp"
+code_fence_block_name = "csharp"
 grammar = "c_sharp"
 path_suffixes = ["cs"]
 line_comments = ["// "]