Fix addSource to allow implementation path to point to a file

[ExE-Boss]

This fixes the bug where:

const transformer = new WebIDL2JS();

transformer.addSource("path/to/Attr.webidl", "path/to/Attr-impl.js");
// ... add the rest, one file at a time. The client is responsible for directory iteration.

await transformer.generate("outputDir");

Would result in outputDir/Attr.js requiring path/to/Attr‑impl.js/Attr.js instead of path/to/Attr‑impl.js, because the wrapper generation code currently assumes that the impl path argument is always a directory.

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This will also make it possible to implement jsdom/jsdom#2835 without renaming Window.js (jsdom/jsdom#2837) by utilising the above mention impl path name fix.

[domenic]

As I expressed in #208 and #160, I'd like us to decide on an API through discussion, before starting any pull requests.