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>

crates/assistant_tooling/src/assistant_tooling.rs

@@ -1,5 +1,9 @@
-pub mod registry;
-pub mod tool;
+mod attachment_registry;
+mod project_context;
+mod tool_registry;
 
-pub use crate::registry::ToolRegistry;
-pub use crate::tool::{LanguageModelTool, ToolFunctionCall, ToolFunctionDefinition};
+pub use attachment_registry::{AttachmentRegistry, LanguageModelAttachment, UserAttachment};
+pub use project_context::ProjectContext;
+pub use tool_registry::{
+    LanguageModelTool, ToolFunctionCall, ToolFunctionDefinition, ToolOutput, ToolRegistry,
+};