Fail fast on invalid Jinja2Template instantiation parameters

[pdelagrave]

Summary

Calling Jinja2Template() with both directory and env shouldn't be allowed. When both parameters were used, the passed env was silently ignored in favor of creating a new one with the provided directory and the deprecated env_options.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Kludex]

This reflects the overrides. 👍

I think we should probably remove this assertion at some point... The type checker is able to catch this anyway.