rustdoc: add tests that validate emitted JS and HTML

[jsha]

For the rustdoc tests, it would be great to validate all emitted JS and HTML in a build of the stdlib. For the JavaScript, we could use node -c to check that it parses. For HTML, we could use the W3C Markup Validator or the newer W3C Nu Html Checker.

[syvb]

Note that Rustdoc currently outputs invalid HTML in some cases according to the Nu validator.

JS files do currently pass node -c.