Add markdown-style em and strong format support

[bacoords]

What?

In the editor, if a user types * or _ before and after a word/phrase, the text is then wrapped in <em> tags. If a user types ** or __ before and after a word/phrase, the text is then wrapped in <strong> tags.

This follows the markdown format, similar to backticks being used for <code>, hash characters for headings, lists, horizontal rule, etc.

Why?

There is inconsistent support for Markdown-style shortcuts currently.

Closes #62243

How?

Updates the format-library package to include support for italic and bold shortcuts.

Testing Instructions

1. Open a post or page.
2. Insert a paragraph block.
3. Start typing and try starting/ending a phrase with *, _ to see italics and **, __ to see bold.

Testing Instructions for Keyboard

Screenshots or screencast

Screen.Recording.2024-06-04.at.8.22.39.AM.mov

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[carolinan]

How do users enter actual *text and **text in their content without enforcing the formatting?
(Not everyone will know about or will want to use the footnotes)

[ellatrix]

I've considered this before and we were worries about false positives, but maybe that worry is questionable. Input rules can be undone with cmd+z, Backspace, and Escape I believe.

[Mamaduka]

Yes, the input rules can be undone via CMD+Z and Backspace.

[carolinan]

That assumes the users knows about it.

[bacoords]

I would just note that the convention of transforming text based on special characters already exists. If a user wants to start a paragraph with a hash # or a hypen - they have this same issue - the block is transformed into a heading or list and they have to learn to use backspace or cmd-z to reverse it. The same for using backticks inline (which can only be reversed with cmd-z, not backspace, I believe).

This would behave similar to the backtick character, in that if you have more than one in a paragraph, it will automatically transform the text in between, but you can undo it with cmd-z. Also it will not automatically transform any text that is copy/pasted or added out of order.