Normative: add globalThis (#702)

 - Adds intrinsic `%GlobalThisValue%`
 - Sets it in `SetRealmGlobalObject`
 - Adds global name `globalThis`

Per tc39/proposal-global#12

spec.html

@@ -7484,8 +7484,7 @@ <h1>GetNewTarget ( )</h1>
       <h1>GetGlobalObject ( )</h1>
       <p>The abstract operation GetGlobalObject returns the global object used by the currently running execution context. GetGlobalObject performs the following steps:</p>
       <emu-alg>
-        1. Let _ctx_ be the running execution context.
-        1. Let _currentRealm_ be _ctx_'s Realm.
+        1. Let _currentRealm_ be the current Realm Record.
         1. Return _currentRealm_.[[GlobalObject]].
       </emu-alg>
     </emu-clause>
@@ -24441,6 +24440,12 @@ <h1>The Global Object</h1>
   <emu-clause id="sec-value-properties-of-the-global-object">
     <h1>Value Properties of the Global Object</h1>
 
+    <emu-clause id="sec-globalthis">
+      <h1>globalThis</h1>
+      <p>The initial value of the `globalThis` property of the global object in a Realm Record _realm_ is _realm_.[[GlobalEnv]]'s EnvironmentRecord's [[GlobalThisValue]].</p>
+      <p>This property has the attributes { [[Writable]]: *true*, [[Enumerable]]: *false*, [[Configurable]]: *true* }.</p>
+    </emu-clause>
+
     <emu-clause id="sec-value-properties-of-the-global-object-infinity">
       <h1>Infinity</h1>
       <p>The value of `Infinity` is *+&infin;* (see <emu-xref href="#sec-ecmascript-language-types-number-type"></emu-xref>). This property has the attributes { [[Writable]]: *false*, [[Enumerable]]: *false*, [[Configurable]]: *false* }.</p>