Fix Dots style Separator in Twenty Twenty, adding charset and alternative text

[sabernhardt]

1. Adds the dots content property and auto height in the theme, with the same selectors they had in block-library styles before WordPress 6.6.
2. Assigns UTF-8 charset to make sure CSS content displays correctly even if the site uses a different character set.
3. Adds alternative text syntax for each content property that is not already empty or none, and leaves the original content property as a fallback for older browsers.
   • dots in the Separator block
   • quotation mark in the Pullquote block
   • em dash in the RSS widget
   • SVG for checkboxes

Props umeshsinghin, narenin

Trac 61787

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Props umeshsinghin, sabernhardt.

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