P2-837 Refactor block validation enum

[johannadevos]

Summary

This PR can be summarized in the following changelog entry:

• [@yoast/schema-blocks] Refactors the BlockValidation enum to group valid, okay and invalid results into three categories.

Relevant technical choices:

• This is a change proposed by @herregroen. The motivation is that it will be easier to write logic based on the three categories of 'validness'.
• I created a new enum value OK, for those validation results that should result in an orange bullet (for reasons other than being a missing recommended block, which has its own value in the enum).
• I simplified the isValidResult function to just check whether the result is < 300 (because all invalid results are 300+). A change from the previous version of the code is that the Skipped result now is considered valid too, whereas previously it was invalid (because it was absent in isValidResult). I think this is not a problem, because we should not see this result anyway.

Test instructions

This PR can be tested by following these steps:

• Test https://github.com/Yoast/javascript/pull/1124 again, following the test instructions there.
• Do a smoke test on the sidebar analysis; do you see the bullets you expect when making changes (for example, adding/removing required and recommended blocks, having an empty required block, having a job title that equals the post title)?

Impact check

• This PR affects the following parts of the plugin, which may require extra testing: *

UI changes

• [ ] This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Quality assurance

• [x] I have tested this code to the best of my abilities
• [ ] I have added unittests to verify the code works as intended

Fixes #

[hansjovis]

Acceptance test: ✅

• Tested https://github.com/Yoast/javascript/pull/1124 again.
• Did a smoke test on the analysis, everything still works as expected.