fix(intl): #5585 — ECMA-402 option/locale validation for the six small constructors

Brings the small Intl constructors (Collator, Segmenter, ListFormat,
RelativeTimeFormat, PluralRules) up to spec on option coercion, locale-list
handling, and a few behavioral gaps. test262 intl402 parity across these six
dirs goes from 241/387 (62%) to 297/387 (77%) — 56 cases fixed, no regressions.

Shared helpers (intl.rs):
- `get_option_string_coerced` / `enum_option_strict` — GetOption with full
  ToString coercion (null → "null", Symbol → TypeError), so `options-*-invalid`
  values are rejected instead of silently defaulting.
- `get_options_object` (GetOptionsObject: non-Object/non-undefined → TypeError)
  and `coerce_options_reject_null` (ToObject: null → TypeError), wired per the
  spec step each constructor uses.
- `locales_from_value` now follows CanonicalizeLocaleList: null → TypeError,
  array-like Objects are iterated (`{0:"DE",length:1}` → `["de"]`), element
  type-checks throw TypeError, and a throwing/Symbol `length` propagates.
- `supportedLocalesOf` validates its `options` arg (localeMatcher) and filters
  the request list through a BestAvailableLocale check, so the result length
  matches (e.g. `zxx` is dropped).
- Per-constructor: read `localeMatcher` (+ `numberingSystem` for RTF) in the
  exact GetOption order the `options-order` / read-order tests assert.

Collator (date_collator.rs):
- NFD-normalize before comparing so canonically-equivalent strings collate 0.
- Resolve `numeric`/`caseFirst`/`collation` from options or the locale's
  `-u-kn`/`-u-kf`/`-u-co` extension keywords; reflect them (plus `usage`,
  `sensitivity`, `ignorePunctuation`, `collation`) in resolvedOptions in spec
  order. `ignorePunctuation` now strips ignorable punctuation in compare.

RelativeTimeFormat / PluralRules (list_relative_plural.rs):
- RTF format/formatToParts use full ToNumber (Symbol → TypeError, object valueOf
  honoured); units are case-sensitive; formatToParts attaches the `unit` field
  to the numeric parts.
- PluralRules reads options in SetNumberFormatDigitOptions order, adds
  `notation`/`compactDisplay`, and `selectRange` throws TypeError for undefined
  or Symbol arguments.

Out of scope (still failing, need CLDR data or larger runtime work): locale-
specific formatting/collation, the prototype accessor-getter shape for
compare/format, %Segments.prototype%.containing, subclassing/new.target, and
@@toStringTag inheritance on instances.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Ralph