Implement addFile(…) and addDirectory(…) Transformer APIs

[ExE-Boss]

Part of #160

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I implemented addFile(…) and addDirectory(…) to match addSource(…), except with an optional impl path argument.

This also 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.

This doesn’t yet implement addFromGlob or a way to feed in dynamically generated partial interface definitions (#188).

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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]

Can you discuss your proposal in #160 first?

[domenic]

Let's close this until we have an agreed-upon design for the API in #160.