fix: Disallow only whitespace between markdown shortcuts delimiters

[rfgamaral]

Please describe your changes

This PR disallows Markdown shortcuts (through typing or pasting) for all marks (Bold, Code, Highlight, Italic, and Strike) when the content between the delimiters is only whitespace.

How did you accomplish your changes

Changed the input and paste Regex for all the marks mentioned above to keep supporting as much as possible from the current syntax, while only disallowing "empty strings" between the delimiters for the Markdown shortcuts of each mark.

Note

The regular expression for all marks follows the exact same pattern, the only difference being the delimiter characters for each mark.

How have you tested your changes

I've used both regex101.com:

And the marks demos to test multiple patterns, here's an example for the Highlight extension:

How can we verify your changes

The same way I did above.

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[bdbch]

LGTM