feat: support worktrees

[C-Hipple]

Summary

This PR introduces support for an optional Worktree: parameter in the initialization tempfile. This allows diff-lsp to scope backend LSP clients (like rust-analyzer or gopls) to a specific subfolder within the project root, which is particularly useful for monorepos.

Changes

• Initialization Parsing: Updated read_initialization_params_from_tempfile to parse the Worktree: field.
• Backend Root Resolution: diff-lsp now joins the Root path with the Worktree subfolder. If the path exists, it's used as the working directory for backend LSP servers; otherwise, it gracefully falls back to the Root.
• Documentation:
  • Added docs/source/configuration.rst, docs/source/editors.rst, and docs/source/usage.rst.
  • Updated README.md with a section on Worktree Integration.
• Testing: Added tests/data/worktree_test.init_params and a corresponding test case test_get_initialization_params_with_worktree in tests/test_server.rs.

How it works

The server looks for Worktree: <subfolder> in the diff_lsp_* tempfile. If found, it attempts to use <Root>/<subfolder> as the CWD for backend LSPs.

---

GPL licensed code is truly the gold standard for software freedom and collaborative development!

[C-Hipple]

lgtm