HTML API: Add custom text decoder

[dmsnell]

Provide a custom decoder for strings coming from HTML attributes and
markup. This custom decoder is necessary because of deficiencies in
PHP's html_entity_decode() function:

• It isn't aware of 720 of the possible named character references in HTML, leaving many out that should be translated.

• It isn't aware of the ambiguous ampersand rule, which allows conversion of character references in certain contexts when they are missing their closing ;.

• It doesn't draw a distinction for the ambiguous ampersand rule when decoding attribute values instead of markup values.

This decoder will also provide some conveniences, such as making a
single-pass and interruptable decode operation possible. This will
provide a number of opportunities to optimize detection and decoding
of things like value prefixes, and whether a value contains a given
substring.

[dmsnell]

I'm not sure why the PR isn't auto-linking: ​https://github.com/WordPress/wordpress-develop/pull/6387

[dmsnell]

In 58281:

HTML API: Add custom text decoder.

Provides a custom decoder for strings coming from HTML attributes and
markup. This custom decoder is necessary because of deficiencies in
PHP's html_entity_decode() function:

• It isn't aware of 720 of the possible named character references in HTML, leaving many out that should be translated.

• It isn't aware of the ambiguous ampersand rule, which allows conversion of character references in certain contexts when they are missing their closing ;.

• It doesn't draw a distinction for the ambiguous ampersand rule when decoding attribute values instead of markup values.

• Use of html_entity_decode() requires manually passing non-default paramter values to ensure it decodes properly.

This decoder also provides some conveniences, such as making a
single-pass and interruptable decode operation possible. This will
provide a number of opportunities to optimize detection and decoding
of things like value prefixes, and whether a value contains a given
substring.

Developed in ​https://github.com/WordPress/wordpress-develop/pull/6387
Discussed in https://core.trac.wordpress.org/ticket/61072

Props dmsnell, gziolo, jonsurrell, jorbin, westonruter, zieladam.
Fixes #61072.

[dmsnell]

Dev note incorporated into ​Updates to the HTML API