Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[x] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

• Moved AST traversal out of Linter and into SourceCode#traverse().
• Updated no-useless-returns, constructor-super, and no-this-before-super to they still work. These rules were also tested against v8.57.0 to ensure that such changes would be backwards compatible.
• Updated the v9 migration guide

Refs #16999

Is there anything you'd like reviewers to focus on?

I deviated from the RFC in how I represented the traversal steps. I realized while implementing that relying on type and phase as string comparisons would be a big performance hit, so I added the kind property as a number (keeping type for easier debugging, though not sure if that's needed) and switched phase to a number. That saves us a lot of string comparisons during traversal.