feat: support explicit comparisons to null for prefer-regexp-test

[DMartens]

Add support for explicit comparisons in the prefer-regexp-test rule:

pattern.exec(string) !== null
// is fixed to
pattern.test(string)

Adding the additional check for !== null was straightforward but adding the fix was not.
But I hope, the code is still understandable.
Fixes #838

[changeset-bot]

🦋 Changeset detected

Latest commit: c57acec

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-regexp	Minor

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[ota-meshi]

Thank you!