Add isValid assertion

[patsy-issa]

Introduces isValid to our assertions, can be utilised in the following way to call checkValidity on the following elements:

assert.dom(HTMLFormElement).isValid();
assert.dom(HTMLInputElement').isValid();
assert.dom(HTMLTextAreaElement).isValid();
assert.dom(HTMLButtonElement).isValid();
assert.dom(HTMLOutputElement).isValid();
assert.dom(HTMLSelectElement).isValid();

[scalvert]

This looks great! Would you also mind updating the docs by running yarn docs, and ensure the output makes sense?

[Turbo87]

👏

[scalvert]

@patsy-issa I hope you don't mind, I went ahead and regenerated the API docs for you for isValid. This is good to go! Thanks again!

[patsy-issa]

@scalvert Awesome, thank you.