project: Allow running multiple instances of a single language server within a single worktree (#22182)

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 `workspaceFolder`s 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 #5108

Release Notes:

- Language servers now track their working directory more accurately.

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Co-authored-by: João <joao@zed.dev>