feat: add support for baseline years

[rviscomi]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

Adds the ability to configure the require-baseline rule to target a Baseline year as an alternative to newly or widely available status

What changes did you make? (Give an overview)

• added the integer type as an alternative to the available configuration property enum
• created a BaselineAvailability class to determine whether a feature is supported based on the config
• added tests that use the year config
• generated feature years in addition to their status ID
• regenerated baseline-data.js
• updated docs
• ran the formatter, which generated an unrelated change to README.md

Related Issues

fixes #78

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

For the rule schema, would you prefer to configure the baseline year as a separate optional property, rather than shoehorning it in with available?

[nzakas]

Thanks for putting this together. I'm not sure how useful this feature is, so let's discuss back on #78 before iterating on the code.

[nzakas]

I just left a few cleanup notes. Overall this looks good.

Do we want to wait to merge #82 before merging this?

[rviscomi]

Thanks @nzakas this is ready for another look

Do we want to wait to merge #82 before merging this?

No preference. Merge conflicts either way 😁

[nzakas]

Almost there, just left a few comments around optimization.

[nzakas]

Double-checking: is this intentional? The max now would be 2026?

Or did you mean this?

Suggested change

	maximum: new Date().getFullYear() + 1,
	maximum: new Date().getFullYear() - 1,

[rviscomi]

Ah, yes this was intentional (see #81 (comment)) but I just realized the maximum is inclusive. My reasoning was that the current year is technically a valid Baseline year, ie https://web.dev/baseline/2025.

Fixed this to use the current year, but let me know if you'd still prefer YYYY - 1.

[nzakas]

Sounds good to me. 👍

[nzakas]

LGTM. Thanks for all of your work on this.