Bundled themes: Stylesheets should be minified in classic themes

[westonruter]

This is a follow-up to #63012 which minified stylesheets in bundled block themes. The same remains to be done for bundled classic themes. This includes dedicated stylesheets for blocks, which would then allow them to be inlined per #58519. See also #63007 which enabled the stylesheets for bundled block themes to be inlined.

Blocking:

• #58519

## Summary
Implements automated CSS minification for 11 classic WordPress themes (Twenty Ten through Twenty Twenty-One) to improve frontend performance. Minified CSS files are conditionally loaded based on the SCRIPT_DEBUG constant.

Initial reference PR: ​https://github.com/WordPress/wordpress-develop/pull/10081

## Changes Made

### Build Infrastructure

• Added cssmin:themes Grunt task for automated CSS minification
• Updated .gitignore to exclude generated .min.css files from version control

### Per-Theme Changes
Each theme received:

• package.json - npm build configuration with cssnano dependencies
• package-lock.json - Locked dependency versions
• contributing.txt - Developer documentation for CSS minification workflow
• Updated functions.php - Conditional loading logic using SCRIPT_DEBUG
• Updated CSS files - Added minification notice comments

Trac ticket: https://core.trac.wordpress.org/ticket/64109

@rutviksavsani it might make sense to open a pull request first just for the Twenty Twenty-One theme to facilitate review and it iron our the kinks. Then once that is reviewed and committed, another PR can be opened to replicate the established pattern from TT1 to the prior classic themes.

That is to say, maybe cherry pick ​https://github.com/WordPress/wordpress-develop/pull/10818/commits/136b585659adaa0a1437b6599b18f16e80a785e8 into a separate PR.

This PR implements the Minification for TwentyTwentyOne theme initially to test out before we convert all of them in ​https://github.com/WordPress/wordpress-develop/pull/10818

Trac ticket: https://core.trac.wordpress.org/ticket/64109

@westonruter As suggested in the other ​PR for minification of stylesheets for themes, Here I have created a separate PR for TT1.

cc @sabernhardt

That is to say, maybe cherry pick ​136b585 into a separate PR.

See ​https://github.com/WordPress/wordpress-develop/pull/10860

@rutviksavsani With r61628 (05e54af), IE support has been removed from the theme and this has introduced merge conflicts. Would you address?

[westonruter]

Punting based on ​feedback from @sabernhardt

[wildworks]

As this ticket has been inactive for a while, I will move the milestone to "Feature Release". Please feel free to move it back if it can be completed by 7.1 beta 1.