Fix email editor styles sneaking into other editors

[Imran92]

Resolves #7228

Proposed Changes

• Moves the import of the css to the inside of a wrapper class to make sure the styles will be applied only for emails

Testing Instructions

1. Activate the Twenty Twenty Three theme.
2. In the site editor, edit the "Blog Home" template.
3. Open DevTools and select one of the elements with the wp-block-site-title class.
4. See that the no styles are coming from the email-editor-style.css file.
5. Open the Email Template in editor
6. Make sure the styles are getting applied as expected
7. Open an email in editor from Sensei LMS -> Settings -> Emails
8. Make sure the styles are getting applied as before and it isn't broken

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[donnapep]

Just wanted to note that the styles are not exactly the same for emails, as now more styles are being applied for them. But they actually look slightly better and this was probably the original intention.