Convert text_justified rule to a JS rule

[pattonwebz]

This PR shifts the text_justified php rule over to a JS equivalent so that it can determine rendered/computed styles to check against.

This is currently set up to align with the description of the PHP rule but not necessarily the code of the PHP rule.

• Only checks against <p>, <div> and <td> elements (as per the user-facing description of the rule, the actual PHP code was checking all elements, though).
• Checks only elements over 500 characters in length.

@amberhinds @SteveJonesDev I have two questions.

• Should the rule actually only check those 3 elements or do we want it to be wider? If so the description of the rule will need a tweak.
• Can we reduce the threshold of the check from 500 to something more like 200 so that it checks when the content exceeds around 2 lines of text rather than the 5 or 6 it currently checks at?

Closes: #634

[amberhinds]

as per the user-facing description of the rule, the actual PHP code was checking all elements, though

This was my mistake. We should check everything - I can edit docs.

I agree with the recommendation to reduce the character count to 200.

[SteveJonesDev]

@amberhinds we'll open an issue to update the docs.

[SteveJonesDev]

@amberhinds, I updated the documentation for this rule: https://equalizedigital.com/accessibility-checker/justified-text/

@pattonwebz before merging can you update the summary?

'summary'   => esc_html__( 'A Text Justified warning appears in Accessibility Checker when text with more than 200 characters on your post or page is styled with justified alignment (text-align:justify). To fix this warning, remove the justified styling from the specified text elements.', 'accessibility-checker' ),

[amberhinds]

Thanks @SteveJonesDev!