Normative: Add variants to Intl.Locale surface area (#960)

Author: gibson042

spec/locale.html

@@ -87,7 +87,9 @@ <h1>
         1. Let _region_ be ? GetOption(_options_, *"region"*, ~string~, ~empty~, GetLocaleRegion(_baseName_)).
         1. If _region_ is not *undefined*, then
           1. If _region_ cannot be matched by the <code>unicode_region_subtag</code> Unicode locale nonterminal, throw a *RangeError* exception.
-        1. Let _variants_ be GetLocaleVariants(_baseName_).
+        1. Let _variants_ be ? GetOption(_options_, *"variants"*, ~string~, ~empty~, GetLocaleVariants(_baseName_)).
+        1. If _variants_ is not *undefined*, then
+          1. If _variants_ cannot be matched by the <code>unicode_variant_subtag</code> Unicode locale nonterminal, throw a *RangeError* exception.
         1. Let _allExtensions_ be the suffix of _tag_ following _baseName_.
         1. Let _newTag_ be _language_.
         1. If _script_ is not *undefined*, set _newTag_ to the string-concatenation of _newTag_, *"-"*, and _script_.
@@ -310,6 +312,16 @@ <h1>Intl.Locale.prototype.toString ( )</h1>
       </emu-alg>
     </emu-clause>
 
+    <emu-clause id="sec-Intl.Locale.prototype.variants">
+      <h1>get Intl.Locale.prototype.variants</h1>
+      <p>`Intl.Locale.prototype.variants` is an accessor property whose set accessor function is *undefined*. Its get accessor function performs the following steps:</p>
+      <emu-alg>
+        1. Let _loc_ be the *this* value.
+        1. Perform ? RequireInternalSlot(_loc_, [[InitializedLocale]]).
+        1. Return GetLocaleVariants(_loc_.[[Locale]]).
+      </emu-alg>
+    </emu-clause>
+
     <emu-clause id="sec-intl.locale.prototype-%symbol.tostringtag%" oldids="sec-Intl.Locale.prototype-@@tostringtag">
       <h1>Intl.Locale.prototype [ %Symbol.toStringTag% ]</h1>