Formatting: Add strip_html_newlines() function

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/5678

This PR adds strip_html_newlines() to wp-includes/formatting.php, a function that strips unnecessary newlines from text nodes in fully marked-up HTML while preserving newlines inside pre, code, kbd, script, and style elements (maybe we can fix the hardcoding and think of something better later but keeping this simple for now)

This is useful for cleaning up imported HTML content before it is processed by wpautop(), which would otherwise convert superfluous newlines into unwanted <br /> elements.

Testing

npm run test:php -- --filter=Tests_Formatting_StripHtmlNewlines

Refreshing and inspired by the patch - https://core.trac.wordpress.org/attachment/ticket/5678/5678.r6633.diff
Props @hansengel

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