Fix Number and Float validation errors, allow underscores, add tests. Fix issue #4718

[merriam]

Please review the following checklist.

• [ n/a] Docstrings on all new or modified functions / classes
• [ tbd] Updated documentation
• [ tbd] Updated CHANGELOG.md (where appropriate)

Here are some changes to the Validation routines:

• Number validator no longer accepts 'nan'.
• Number regex no longer accepts 'e' as first character of a number.
• Integers no longer accept scientific notation. That is, "1.23e2" happens to resolve to an integer if first viewed as a float, but cannot be cast with int(value).
• Integers and floats allow underscores, per the Python standard. That is, "1_000_000" is a valid integer and can be typed in a "integer" type field. "12." cannot be typed or validated.

The tbd on documentation: does this warrant a note in the CHANGELOG.md? Also, I started cleaning up the Input Widet documentation and its worth a different commit.

[darrenburns]

LGTM

@merriam Any change which could affect a developer using Textual or an end-user using a Textual app warrants a changelog entry. Since this affects the behaviour of the Input, we'd need a changelog entry for it.

[merriam]

Sorry, I hadn't realized it was waiting for CHANGELOG. Anything else needed?