Formatting: introduce esc_attr_name() for HTML attribute name sanitization

[SainathPoojary]

The HTML5 spec allows arbitrarily named attributes (e.g., data-*), but WordPress lacked a dedicated function to escape dynamically generated attribute names. Developers sometimes incorrectly used esc_attr() for this, which only escapes attribute values.

This PR introduces esc_attr_name() in formatting.php to resolve this. It uses a strict allowlist regex ([^a-zA-Z0-9_.:\[\]-]+/u) to strip any characters not explicitly permitted in an HTML attribute name per the HTML5 specification (such as spaces, quotes, control characters, <>, =, etc.).

Trac ticket: #43010

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Props sainathpoojary, irozum, huzaifaalmesbah.

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[huzaifaalmesbah]

A couple of edge cases to consider here:

1. Unicode Support: The regex restricts attribute names to ASCII only, but the WHATWG HTML specification allows valid non-ASCII/Unicode characters in attribute names. For instance, data-ñame currently gets stripped to data-ame.

2. Empty Return Value: If $text contains only invalid characters (like spaces or quotes), esc_attr_name() returns an empty string "". When concatenated as <div " . esc_attr_name( $attr ) . '="val">, this results in malformed HTML (<div ="val">).