Allow underscores as separators in the math builtin

[andmis]

Description

This change allows underscores as visual separators in the math builtin, so math 1_000 + 2_000 will return 3000. We allow leading, trailing, and multiple underscores. See the documentation of the fish_wcstod_underscores function in the PR.

See also #8607.

Fixes issue #8496.

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[krobelus]

looks really nice, I only have nitpicks

[andmis]

Thanks for the review. A missed null pointer check isn't a nitpick. :)

I've made the changes you requested -- one thing is that I added a changelog entry and I'm not sure about where I put it, so before merging someone should double-check that.

[zanchey]

I want to hold this until after 3.4.0 is out as we're doing the final QA for the release now.

[andmis]

Yep that makes sense -- we'll just have to move the changelog entry before merging (it's under 3.4 right now).

[faho]

Merged now, thanks!