MessageValue should not have a locale property

[stasm]

The MessageValue should not have a locale property. That's what Parts are for.

Consider {$count :number}. The number stored in $count may be decorated with formatting options like minimumFractionDigits etc., but it's not in any particular locale just yet. It's the locale of the message (or the placeholder if we allow overriding it) that decides how $count will be formatted.

There's nothing inherently language-specific about the mathematical value of 5 :) Same goes for many string literals, like user names or Wi-Fi network names.

I understand that we need the locale somewhere for toString and toParts to work. I think we should just pass it as argument: toString(locale). Perhaps also call it toLocaleString() for consistency with other JS APIs.

Alternatively, we can consider passing in a larger structure: toString(fmtctx). Such "formatting context" may be useful if we want to allow implementations to cache formatters.

(A formatting context should be passed into formatting methods rather than be referenced by a property of MessageValue to help avoid memory leaks and facilitate IPC use-cases.)

Originally posted by @stasm in #22 (comment)