[Proposal] Add "String Encoding Names" proposal.

[YOCKOW]

This proposal allows String.Encoding to be converted to and from various names.

For example:

print(String.Encoding.utf8.ianaName!) // Prints "UTF-8"
print(String.Encoding(ianaName: "ISO_646.irv:1991") == .ascii) // Prints "true"

---

Implementation: #1286

[YOCKOW]

@itingliu

I think #1029 implies legacy encodings are still necessary in some areas.
While this proposed feature doesn't support some legacy encodings, this feature will make it easy to implement more string conversions since we already have the string converter that wraps ICU APIs and that requires IANA Charset Names to create an instance of naive ICU converter.

I mean I believe this feature is useful not only as public properties but also for internal use.
I hope this proposal will come up to the official process.