feat(meta): disallow invalid rel attribute keywords

src/elements/__snapshots__/html5.spec.ts.snap

src/elements/html5.ts

test-files/elements/a-invalid.html

test-files/elements/a-valid.html

test-files/elements/area-invalid.html

test-files/elements/area-valid.html

test-files/elements/form-invalid.html

test-files/elements/form-valid.html

test-files/elements/link-invalid.html

test-files/elements/link-valid.html