fix: make LangKit data paths configurable

[extrasmall0]

Summary

• allow LANGKIT_DATA_HOME to override where LangKit stores downloaded artifacts
• fall back to a user cache directory when the package directory is not writable (common in containers)
• allow LANGKIT_INJECTIONS_BASE_URL / LANGKIT_DATA_FOLDER overrides and document them in the README
• add tests for the new path-selection behavior

Testing

• python3 -m venv .venv && . .venv/bin/activate && pip install -q pytest whylogs pandas && pytest langkit/tests/test_config_paths.py -q

Closes #322

[extrasmall0]

Quick status refresh after another pass against current main: this is still a narrow path-selection fix for environments where the installed package directory is read-only. The default behavior stays the same when that location is writable, and the fallback only kicks in when writes there are not possible. The tests remain limited to data-path resolution rather than changing analyzer behavior. If there is a preferred config surface or naming for this, I am happy to adjust the patch quickly.