Normative: Follow CLDR schema and rebase/modernize

[littledan]

• Specify that a CLDR-style schema* and ICU-style logic for how the pattern is chosen based on the dateStyle and timeStyle parameters, rather than being entirely implementation-defined. Closes #3
• Rebase against master and modernize the specification editorially; remove unchanged specification sections. Closes #5

This patch implies that OS options are not taken into account when using dateStyle/timeStyle; the data instead comes from a source like CLDR.

• The schema here omits that the patterns are calendar-specific. The fix is out for review in https://github.com/tc39/ecma402/pull/227 .

[littledan]

cc @jungshik @FrankYFTang @zbraniecki @ms2ger

[littledan]

Given that this proposal is just at Stage 1, I'll land the PR so that it'll be easier to review the result. We'll be reviewing it at a future Intl meeting.