Normative: Update time zone ID requirements

This PR resolves tc39#825 by adding spec text that defines how ECMA-402
implementations should decide which IANA time zone IDs should be
primary or non-primary.

This PR implements "Option C" in tc39#825 by deterministically defining
ECMAScript's exceptions from the IANA Time Zone Database's defaults,
and then pointing implementers at ICU as a convenient implementation
of those exceptions.

This PR also accommodates to web reality by aligning the 402 spec text
with the existing behavior of ICU4C which is used by most (all?) major
ECMAScript engines to implement time zone features.

This PR is stacked on top of an editorial PR to align ECMA-402's
time-zone-related spec text with ECMA-262.