Editorial: refactor time zone offset handling (prepare for #2607)

[justingrant]

This PR refactors spec text and polyfill code for time zone offsets, splitting the handling of offsets used as time zone identifiers from offsets used for ZDT's offset property (both parsing and output), and for parsing of Instant and the RFC3339 portion of ZDT strings.

This split will prepare for the normative changes in #2607 and will make the normative PR easier to review.

Here's more details about what's in this PR:

• Moves offset string grammar into abstractops.html, remove it from mainadditions.html
• Splits the |UTCOffset| production into |UTCOffsetMinutePrecision| and |UTCOffsetSubMinutePrecision|.
• Replaces the pattern of calling IsTimeZoneOffsetString, throwing if it's false, and then calling ParseTimeZoneOffsetString. Instead, offset-parsing AOs (like other Temporal parsing AOs) will throw if they're provided with an invalid string, which lets us replace two AO calls and an If statement with a single call to a parsing AO.
• Splits ParseTimeZoneOffsetString into two new AOs for parsing ISO string offsets and offset time zone identifiers, respectively. Both return Records instead of primtives:
  • ParseUTCOffsetString returns { [[OffsetNanoseconds]: an integer, [[HasSubMinutePrecision]]: a Boolean }
  • ParseTimeZoneIdentifier returns { [[Name]]: a String or ~empty~, [[OffsetNanoseconds]]: an integer or ~empty~}
  • If the input string isn't valid, both new AOs throw a RangeError.
• Renames IsTimeZoneOffsetString to IsOffsetTimeZoneIdentifier.
• Updates ecmarkup to a version that supports <ins> and <del> in structured headers in order to support the renames above.
• Aligns offset parsing in the polyfill more closely to the spec text.
• Adds a note to ISO grammar about how we extend the standard to allow 9 digits for instant string offsets.

[codecov]

Codecov Report

Merging #2621 (90f0ab2) into main (3bf0cfe) will increase coverage by 0.00%.
The diff coverage is 98.27%.

@@           Coverage Diff           @@
##             main    #2621   +/-   ##
=======================================
  Coverage   95.98%   95.98%           
=======================================
  Files          20       20           
  Lines       11572    11574    +2     
  Branches     2198     2201    +3     
=======================================
+ Hits        11107    11109    +2     
  Misses        401      401           
  Partials       64       64           

Impacted Files	Coverage Δ
polyfill/lib/intl.mjs	97.84% <0.00%> (ø)
polyfill/lib/ecmascript.mjs	98.26% <100.00%> (+<0.01%)	⬆️
polyfill/lib/regex.mjs	100.00% <100.00%> (ø)
polyfill/lib/timezone.mjs	100.00% <100.00%> (ø)
polyfill/lib/zoneddatetime.mjs	100.00% <100.00%> (ø)