Define __all__ to re-export classes

[jacobcheatley]

This makes:

import jinjax

# And then using...
jinjax.JinjaX
# Or some other symbol

behave properly for type checkers like pyright by conforming to the PEP561 / type information in libraries spec. Linters also stop complaining, so you can remove the #noqa.

Without the change, pyright reports: error: "JinjaX" is not exported from module "jinjax" (reportPrivateImportUsage)

This can be worked around by ignoring the reportPrivateImportUsage rule, but it's nice to have.

An alternative to setting __all__ would be redundant aliases (e.g. from .jinjax import JinjaX as JinjaX) but I figured those were less nice.

microsoft/pyright#2639 (comment)