ProperEscapingFunction: improve "action" match precision

[jrfnl]

In VIPCS 2.2.0, checking for the use of esc_url() for "action" HTML attributes was introduced in PR #575 in response to issue #554.

As it is not inconceivable that "action" is used as a suffix for custom HTML attributes - like "data-action"- for which the value does not necessarily has to be a URL, the check for the "action" HTML attribute should make sure it is the complete attribute name and not used as a suffix for a custom attribute name.

This change contains a minor refactor of the code which examines the content of the previous text string.

Instead of looping over the various lists and doing a substr() on the same content 25 times, it will now use a regular expression to gather the necessary information to throw the right errors in one go.

Includes unit tests.

Fixes #669

Note: This PR removes two private properties which were introduced in 624/VIPCS 2.3.0 and two public methods.

The removal of the properties is safe. The removal of the public methods could be considered a BC-break as these methods were public, though they never should have been.

If so preferred, the public methods could be deprecated instead and remain in the code base as dead code/emptied out methods until the next major release upon which they could be removed.

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New commits since pulling

ProperEscapingFunction: fine-tune attribute regex

This adds test cases with:

• No space before "action" as it is at the start of the line in a multi-line text string.
• A tab before "action".

... and makes minor adjustments to the regex to safeguard handling these cases correctly.

ProperEscapingFunction: deprecate, don't remove

... the public methods and as those use the private properties and extending sniffs may rely on the functionality of the public methods, we can't removed the private properties yet either, so deprecating those too.

[rebeccahum]

Very clever! I love how you were able to eliminate the looping over and kill many birds with one stone.