Reorganize constants in packages/core

[tlylt]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Overview of changes:
An initial attempt to refactor existing code within packages/core. To keep changes manageable and easy to review, this PR only addresses the following:

• Relocate constants back to relevant files if they are not used in more than one place
• Remove unused constants
• Use newlines to visually group constants better
• Minor typo fix
• Remove outdated comments
• Use defined constants if available
• Extract HTML string to constant

Anything you'd like to highlight/discuss:
Previously, an attempt was made to separate constants into individual files in #949. While there is nothing wrong with the approach, I feel that re-evaluating the existing constants and shifting them into appropriate locations could help reduce unnecessary exports and set the stage for further refactoring. There could be personal preference involved so up for discussion on whether this PR improves the codebase.

Testing instructions:
nil, this refactor should not affect existing functionalities and hence CI checks should be sufficient

Proposed commit message: (wrap lines at 72 characters)
Reorganize constants in packages/core

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

[ang-zeyu]

I feel that re-evaluating the existing constants and shifting them into appropriate locations could help reduce unnecessary exports and set the stage for further refactoring. There could be personal preference involved so up for discussion on whether this PR improves the codebase.

Agreed here, this reorganization looks good! 👍