Consider adding TimeZone constructor from IANA

[sffc]

Currently if you want to create a TimeZone from an IANA ID, you need to know to create an IanaParser.

It would be more discoverable, especially on FFI, if TimeZone itself had a function with the following signature

impl TimeZone {
    pub fn try_from_iana_str(iana_str: &str, parser: IanaParserBorrowed) -> Self { ... }
}

If we add this constructor, we should make it the default constructor for the WASM Demo.