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.
Move the remaining LibJS primitive string users to UTF-16 views and
strings. Remove the primitive string UTF-8 accessors and byte-string
coercion paths so new callers cannot rely on the old storage model.
We now clear the computed font cache whenever a `@font-feature-values`
rule is added, removed, or has one of it's descriptors modified.
This isn't observable yet since we don't actually respect
`@font-feature-values` rules, but that will come in a later commit
