Normative: Update String toLocale{Lower,Upper}Case to ResolveLocale with best-fit matching

Fixes tc39#896

Author: gibson042

spec/locale-sensitive-functions.html

@@ -79,13 +79,11 @@ <h1>
         </dl>
         <emu-alg>
           1. Let _requestedLocales_ be ? CanonicalizeLocaleList(_locales_).
-          1. If _requestedLocales_ is not an empty List, then
-            1. Let _requestedLocale_ be _requestedLocales_[0].
-          1. Else,
-            1. Let _requestedLocale_ be DefaultLocale().
           1. Let _availableLocales_ be an Available Locales List which includes the language tags for which the Unicode Character Database contains language-sensitive case mappings. If the implementation supports additional locale-sensitive case mappings, _availableLocales_ should also include their corresponding language tags.
-          1. Let _match_ be LookupMatchingLocaleByPrefix(_availableLocales_, « _requestedLocale_ »).
-          1. If _match_ is not *undefined*, let _locale_ be _match_.[[locale]]; else let _locale_ be *"und"*.
+          1. Let _opt_ be the Record { [[localeMatcher]]: *"best fit"* }.
+          1. Let _relevantExtensionKeys_ be a new empty List.
+          1. Let _r_ be ResolveLocale(_availableLocales_, _requestedLocales_, _opt_, _relevantExtensionKeys_).
+          1. Let _locale_ be _r_.[[Locale]].
           1. Let _codePoints_ be StringToCodePoints(_S_).
           1. If _targetCase_ is ~lower~, then
             1. Let _newCodePoints_ be a List whose elements are the result of a lowercase transformation of _codePoints_ according to an implementation-derived algorithm using _locale_ or the Unicode Default Case Conversion algorithm.