Add attribute reflection extended attributes

A number of extended attributes are added which trigger IDL attribute reflection:

- [Reflect] and [Reflect=string] cover basic content attribute reflection cases.
- [ReflectSetter] and [ReflectSetter=string] covers content attribute reflection only for setters.
- [ReflectURL] and [ReflectURL=string] covers reflection of USV attributes, which represent URLs.
- [ReflectNonNegative] and [ReflectNonNegative=string] covers reflection of long attributes that should be limited to only non-negative numbers.
- [ReflectPositive] and [ReflectPositive=string] covers reflection of double and unsigned long attributes which should be limited to positive numbers.
- [ReflectPositiveWithFallback] and [ReflectPositiveWithFallback=string] covers reflection of double and unsigned long attributes which should be limited to positive numbers with fallback.

Additionally:

- [ReflectRange=(integer, integer)] can be used alongside others on unsigned long attributes that should be clamped to a range.
- [ReflectDefault=number] can be used alongside others on double, long and unsigned long attributes that should be reflected with a default value.

Corresponding Web IDL PR: whatwg/webidl#1503.

Author: lukewarlow