Allow extracting (deeply) nested calls in Python and Javascript

[dylankiss]

Currently the Python extractor does not support deeply nested gettext calls (deeper than as a direct argument to the top-level gettext call).

e.g.

_("Hello %s", _("Person"))
_("Hello %s",
  random_function(", ".join([_("Person 1"), _("Person 2")])))

The extraction code was refactored quite a bit to simplify the flow and support this use-case.

Currently the Javascript extractor does not support nested gettext calls at all.

The extraction code was refactored a bit to resemble the Python code as much as possible and support this use-case.

Fixes #1125 (meanwhile also fixes #1123)

[dylankiss]

During the refactor, the order of extraction was also changed, as you can see in this test:
https://github.com/python-babel/babel/pull/1127/files#diff-c74d633b5cd37350f5a10b2697475119ba1db4f541eeccec744f7d79ab99d6c1R437-R452

It is now the same as the extraction order of xgettext. Also the comments extraction was fixed to be the same as xgettext and apply to all gettext calls (also nested ones) on the same line.
Nested translator comment with nested gettext calls are also supported now, just like with xgettext.

e.g.

# NOTE: Main Comment
_("Hello %s",
    # NOTE: Nested Comment
    _("Nested Gettext")
)

Both terms would get their right comment extracted.

[tomasr8]

Not saying this is not worth fixing, but out of curiosity, do nested gettext calls actually come up often? I don't think I've ever come across one..

[dylankiss]

@tomasr8 In our own codebase with lots of developers, people assume it works and it happens from time to time that they add in nested gettext calls. Even the deeply nested ones happen, like this example: https://github.com/odoo/odoo/pull/149921/files#diff-e073b7fa9d45d46ba8d7f011257b0e77e1f87bf47982abc63dd618ff05dddb1aL267-L268
I think it deserves a fix, meanwhile also fixing some other small issues 🤷

[dylankiss]

UPDATE: I added an extra commit to also allow nested calls in the Javascript extractor. If it's better to open a separate PR for that, no problem.