fix: improve classic editor detection

[HardeepAsrani]

Summary

This improves the Classic Editor detection for the Instant Sharing feature.

Will affect the visual aspect of the product

YES/NO

Screenshots

Test instructions

• This is going to be a bit complex with a lot of variants:
• Make sure the Instant Sharing appears regardless of the type of Editor if the Custom Post Type is enabled.
• Make sure the right kind of Instant Sharing appears based on if the Post Type is using Classic or Block editor.
• Make sure the right kind appears regardless if the post is new, old, switching from classic to old, or old to classic. Regardless if Classic Editor plugin is or isn't installed.
• If Classic Editor is installed, make sure right kind of Instant Sharing options appear based on the current editor, regardless of Classic Editor settings specified in Options > Reading settings, as well as User Profile settings.

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes https://github.com/Codeinwp/tweet-old-post-pro/issues/610.

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