[cvas22]

Fixes #538.

get_undeclared_template_variables() only returns the top level name, so {{ foo.bar }} gives foo. I added a nested=True option that returns the dotted paths instead: foo.bar, and foo.baz for {{ foo['baz'] }}. The default stays the same, so nothing changes for existing callers, and the context filter works with both.

Notes

Only attribute and string-key lookups are followed. Something like items[0] still gives items, since there's no sensible path for an index.

I added tests/get_undeclared_variables_nested.py, which builds its own template and runs fine with the existing get_undeclared_variables.py test.