Remove erroneous *.js pattern for extra_javascript in schema.json

[vorburger]

Since mkdocs/mkdocs#92 for mkdocs/mkdocs#91, an extra_javascript item does not necessarily have to end in *.js; e.g. in enola-dev/enola#669 I have an extra_javascript: - https://unpkg.com/mustache@latest, which this flags up as wrong - although it's not (it works great); ergo it's better to remove this constraint.

[squidfunk]

Thanks for the PR. I would not call it erroneous, since the URL you use is not the final resolved URL – it's redirecting to the final URL which contains the .js extension. However, I agree that we can remove that restriction.