Fix dashicons arrows in RTL

[sabernhardt]

Commits such as [60806] used the alternative text syntax for CSS content, but the ​Gruntfile does not account for that when switching the direction of the arrows for RTL stylesheets.

As a result, the fallback for old browsers is changed correctly while the modern syntax gives the wrong arrow.
​Example in toolbar:

#wpadminbar .menupop .menupop > .ab-item .wp-admin-bar-arrow:before {
	content: "\f141";
	content: "\f139" / '';
}

[sabernhardt]

66020.diff​ is not very polished, but it removes the empty alternative text syntax before comparisons and then adds it back at the end.

In RTL admin screens some dashicons arrows point the wrong way, with the old-browser fallback flipped correctly and the modern value not flipped.

The swap-dashicons-left-right-arrows rtlcss plugin in Gruntfile.js compares each content value against the six arrow glyphs by exact string equality, so a declaration written with alternative text, as in content: "\f141" / '';, never matches. This sets the alternative text aside before the comparison and restores it afterwards.

Seventeen declarations across seven generated stylesheets change, including the toolbar submenu arrows named in the ticket.

Trac ticket: https://core.trac.wordpress.org/ticket/66020

## Use of AI Tools
AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5
Used for: diagnosing the rtlcss value comparison and drafting the Gruntfile change. All changes were reviewed and validated by me.

[lancewillett]

In 63885:

Build/Test Tools: Correct RTL arrows with alternative text.

Set aside CSS content alternative text before matching directional Dashicons, then restore it after swapping the glyph. This keeps modern declarations consistent with their legacy fallbacks in generated RTL stylesheets.

Developed in: ​https://github.com/WordPress/wordpress-develop/pull/13361

Props khokansardar, sabernhardt, joedolson.
Fixes #66020.