Update bindings generator to support default ByteString values in a dictionary

[malisas]

The Web IDL ByteString spec previously said:

There is no way to represent a constant ByteString value in IDL.

But in response to an issue I filed, it has now been changed to say:

There is no way to represent a constant ByteString value in IDL, although ByteString dictionary member and operation optional argument default values can be specified using a string literal.

Conclusion: Servo's bindings generator should be updated to support default ByteString values in a dictionary.

More about string literals: here
This blocks #11896

[Ms2ger]

@malisas: are you planning to work on this? Feel free to ping me or jdm if you need help.

[malisas]

Sure, I'll give it a shot!

[jdm]

There will be two parts to this - allowing default values for ByteStrings in the parser, and generating appropriate Rust code for ByteString values with default values in the code generator. As a good testcase for this work, I suggest adding a ByteString dictionary member with a default value to TestBinding.webidl, as well as a method that accepts an optional ByteString argument with a default value.

Code: components/script/dom/bindings/codegen/parser/WebIDL.py, components/script/dom/bindings/codegen/CodegenRust.py

[jdm]

Relevant code in CodegenRust.py is in the isDOMString() and isUSVString() branches of getJSToNativeConversionInfo.