Support adding dynamic IDL strings to Transformer

[ExE-Boss]

This is a pain point I’ve encountered while trying to convert cssstyle to use WebIDL2JS.

I’m currently forced to generate ./src/CSSStyleDeclaration‑properties.webidl at build time, which contains the generated partial interface attributes:

• https://drafts.csswg.org/cssom/#ref-for-cssstyledeclaration④
• https://drafts.csswg.org/cssom/#ref-for-cssstyledeclaration⑤
• https://drafts.csswg.org/cssom/#ref-for-cssstyledeclaration⑥

[TimothyGu]

For reference, Blink just uses a getter/setter rather than implementing each attribute individually: https://source.chromium.org/chromium/chromium/src/+/master:third_party/blink/renderer/core/css/css_style_declaration.idl;l=1;drc=fdd99242518b07d1d48e6f8453cbb7963a27cb6a.

Gecko also has an IDL generation step: see https://searchfox.org/mozilla-central/source/dom/bindings/GenerateCSS2PropertiesWebIDL.py.

[domenic]

Generating IDL at build time, like Gecko, seems like the right approach to me.

[ExE-Boss]

I also generate them at build time, but I’d like to skip the writing to disk part.

[domenic]

That makes some sense, and perhaps should feed in to #160.