INTERPRETING.md: Add $262.gc() and "host-gc-required" feature (#2208)

INTERPRETING.md

@@ -63,6 +63,7 @@ properties of the global scope prior to test execution.
         5. Let status be ScriptEvaluation(s).
         6. Return Completion(status).
 
+  - **`gc`** - a function that wraps the host's garbage collection invocation mechanism, if such a capability exists. Must throw an exception if no capability exists. This is necessary for testing the semantics of any feature that relies on garbage collection, e.g. the `WeakRef` API.
   - **`global`** - a reference to the global object on which `$262` was initially defined
   - **`IsHTMLDDA`** - (present only in implementations that can provide it) an
     object that 1) has an [[IsHTMLDDA]] internal slot, and 2) when called with

features.txt

@@ -218,3 +218,4 @@ WeakSet
 # object, go here.
 
 IsHTMLDDA
+host-gc-required