feat: replace manual config textarea with CodeMirror JSON editor

[HardeepAsrani]

Summary

Initialize CodeMirror on the manual configuration textarea with syntax highlighting, line numbers, bracket matching, and auto-close brackets

Will affect visual aspect of the product

YES

Screenshots

Test instructions

• Make sure it Manual Config field works as before and Syntax Highlighting is done properly.

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 #509

[pirate-bot]

Plugin build for dc2c21a is ready 🛎️!

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