Use appendItem to produce glue patterns

[Manishearth]

Currently we hardcode some glue patterns as just being space-separated

icu4x/provider/source/src/datetime/neo.rs

Lines 547 to 554 in 6c14cef

	GlueType::DateZone => {
	// TODO: Use proper appendItem here
	"{1} {2}"
	}
	GlueType::TimeZone => {
	// TODO: Use proper appendItem here
	"{0} {2}"
	}

This is, most of the time, the right thing, but there are presumably cases where it isn't and we should use the timezone appendItem pattern instead:

https://github.com/unicode-org/cldr-json/blob/2da48b2aca08a2df9bd004953ec877f3f0b993e4/cldr-json/cldr-dates-full/main/en/ca-gregorian.json#L409-L420

This is specced in https://unicode.org/reports/tr35/tr35-dates.html#Missing_Skeleton_Fields