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

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