project: Allow running multiple instances of a single language server within a single worktree (#23473)

This PR introduces a new entity called Project Tree which is responsible
for finding subprojects within a worktree;
a subproject is a language-specific subset of a worktree which should be
accurately tracked on the language server side. We'll have an ability to
set multiple disjoint workspaceFolders on language server side OR spawn
multiple instances of a single language server (which will be the case
with e.g. Python language servers, as they need to interact with
multiple disjoint virtual environments).
Project Tree assumes that projects of the same LspAdapter kind cannot
overlap. Additionally project nesting is not allowed within the scope of
a single LspAdapter.

Closes https://github.com/zed-industries/zed/issues/5108
Re-lands #22182 which I had to revert due to merging it into todays
Preview.

Release Notes:

- Language servers now track their working directory more accurately.

---------

Co-authored-by: João <joao@zed.dev>

crates/refineable/derive_refineable/Cargo.toml

@@ -16,4 +16,4 @@ doctest = false
 [dependencies]
 syn = "1.0.72"
 quote = "1.0.9"
-proc-macro2 = "1.0.66"
+proc-macro2 = "1.0.93"