feat: toBeValid supports <form/> element

[jsphstls]

What:

toBeValid and toBeInvalid currently support checks against input elements by calling checkValidity().

also responds to checkValidity().

Why:

Discussed in this issue:
#125

How:

Added 'form' to the list of tags that can be checked by toBeValid and toBeInvalid.

Checklist:

• [ x] Documentation
• Tests
• Updated Type Definitions "N/A"
• Ready to be merged

[gnapse]

@jsphstls this looks good. Thanks again!

I wanted to ask you to update the README a little bit to reflect that not only form elements, but the form itself can also be check for validity? At least mention in the README doc for toBeValid and toBeInvalid that an entire form can be checked. No need to add html examples of it, but do it if you feel like it.

[jsphstls]

Let me know how that looks. I noticed that the README example closely matched the test cases, so I changed the invalid element example to be simpler to read.

[gnapse]

🎉 This PR is included in version 4.1.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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