Prevent Registration of Invalid Shortcode Names

[miqrogroove]

All versions of the Shortcode API have allowed registration of any shortcode name, even though certain characters would be impossible to use inside of a shortcode.

In addition to specifying which shortcode delimiters are reserved, we also need to prevent any use of HTML special characters in shortcode names.

See ​https://make.wordpress.org/core/2015/09/29/shortcode-roadmap-draft-two/

[miqrogroove]

Adds unit tests.

[miqrogroove]

Style improvement.

[wonderboymusic]

In 34745:

Shortcodes: prevent registration of invalid shortcode names.

Adds unit tests.

Props miqrogroove.
Fixes #34090.

[pento]

In 35428:

Shortcodes: Tweak the error messages added in [34745].

See #34090.