Centralize project context provided to the assistant (#11471)

This PR restructures the way that tools and attachments add information
about the current project to a conversation with the assistant. Rather
than each tool call or attachment generating a new tool or system
message containing information about the project, they can all
collectively mutate a new type called a `ProjectContext`, which stores
all of the project data that should be sent to the assistant. That data
is then formatted in a single place, and passed to the assistant in one
system message.

This prevents multiple tools/attachments from including redundant
context.

Release Notes:

- N/A

---------

Co-authored-by: Kyle <kylek@zed.dev>

Cargo.lock

@@ -411,10 +411,17 @@ name = "assistant_tooling"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "collections",
+ "futures 0.3.28",
  "gpui",
+ "project",
  "schemars",
  "serde",
  "serde_json",
+ "settings",
+ "sum_tree",
+ "unindent",
+ "util",
 ]
 
 [[package]]