fix: support pygls v2 LanguageServer import

[nisedo]

Context / Bug

slither-lsp imports LanguageServer from pygls.server.

In pygls>=2.0, LanguageServer was moved to pygls.lsp.server, so the import fails and the LSP crashes on startup with:

ImportError: cannot import name 'LanguageServer' from 'pygls.server'

Impact

Any environment that resolves pygls>=2 (e.g., uv tool installs) fails to start slither-lsp, causing the VSCode extension to repeatedly restart the server and never initialize.

Fix

Add a backward‑compatible import shim that tries the old path first and falls back to the new path:

try:
from pygls.server import LanguageServer
except ImportError:
from pygls.lsp.server import LanguageServer

Applied consistently in:

- slither_lsp/app/slither_server.py
- slither_lsp/app/logging/lsp_handler.py
- slither_lsp/app/request_handlers/analysis/get_detector_list.py
- slither_lsp/app/request_handlers/analysis/get_version.py
- slither_lsp/app/request_handlers/compilation/get_command_line_args.py

### Why this approach

- Preserves compatibility with pygls<2 without breaking existing environments.
- Unblocks pygls>=2 without pinning dependencies.
- Minimal, low‑risk change: only import paths, no behavior changes.

### Testing

Manual: verified that slither-lsp starts with pygls>=2 after the change (previously crashed).