numfmt: honor LC_NUMERIC for decimal separator

[pocopepe]

Fixes #11927

Fixing hardcoded '.' as decimal separator with locale separator via locale_decimal_separator(), using it for both input parsing and output formatting

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/csplit/csplit-heap is now being skipped but was previously passing.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.
Congrats! The gnu test tests/unexpand/bounded-memory is now passing!

[cakebaker]

Thanks for your PR!