PR #3869 added support for `<script type="module">` & some support for
the `nomodule` attribute but with no tests for `nomodule` and with the
attribute only respected on inline scripts. This commit adds support for
source-based scripts as well. It also adds tests for `nomodule`, including
making sure legacy browsers execute such scripts as they'd natively do - that's
the whole point of `nomodule` scripts, after all.
Fixes gh-4281
Closes gh-4282
Ref gh-3871
Ref gh-3869
