Member predicate Folder::getJavaScriptFile

Gets the file in this folder that has the given stem and any of the supported JavaScript extensions.

If there are multiple such files, the one with the “best” extension is chosen based on a prioritized list of file extensions.

js files are given less preference than files that compile to js, to ensure we pick the original source file rather than its compiled output.

HTML files will not be found by this method.