Add explicit-heading rule

[TylerJDev]

Adds explicit-heading rule. This rule disallows the following:

• Using Heading without providing a heading level via as, (e.g. <Heading>)
• Using as for any element that is not a heading (h1-h6)

Additional context: #57.

[changeset-bot]

🦋 Changeset detected

Latest commit: 4560876

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
eslint-plugin-primer-react	Major

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[khiga8]

I can review more carefully tomorrow, but do you think this is considered an accessibility rule, and would we want to include this rule in the accessibility scorecard?

If so, we should prefix the name with a11y- which would allow us to flag it as an accessibility rule.

[khiga8]

I have one open question about prefixing this lint rule with a11y- so we can identify it as an accessibility-related rule more easily and eventualy track it in our scorecard. (We're still figuring out the rollout system around that in https://github.com/github/accessibility/issues/3868).

Aside from that, I have no concerns. This is awesome! 🎉

[TylerJDev]

@khiga8 Yup I can definitely add that!

[khiga8]

Great work 🎉

[broccolinisoup]

Sorry I missed this! Thanks for a new shiny rule 🤩 Only thing I would say is that we should add this rule to the README.md.