Produce JS-visible string results as UTF-16 at their source, including
numeric formatting, BigInt and BigFraction formatting, URI encoding,
console formatting, parser errors, regular expression errors, Intl and
Temporal records, LibUnicode locale boundaries, and LibWeb bindings.
Handle fractional radix formatting through the UTF-16 builder view.
CustomStateSet directly selected the style invalidation reason used when
its JS-visible set is modified. Move that mapping into
CSS::Invalidation::CustomElementInvalidator.
This keeps the custom-state container focused on its set contents while
CSS invalidation owns the style work required by :state() selectors.