Bug 1688879 - Part 3: Parse and register an import map. r=jonco,yulia

Implement
https://wicg.github.io/import-maps/#parse-an-import-map-string,
and
https://wicg.github.io/import-maps/#register-an-import-map

Differential Revision: https://phabricator.services.mozilla.com/D142071

js/loader/ScriptLoadRequest.h

@@ -181,6 +181,7 @@ class ScriptLoadRequest
       mozilla::MaybeOneOf<JS::SourceText<char16_t>, JS::SourceText<Utf8Unit>>;
 
   bool IsModuleRequest() const { return mKind == ScriptKind::eModule; }
+  bool IsImportMapRequest() const { return mKind == ScriptKind::eImportMap; }
 
   ModuleLoadRequest* AsModuleRequest();
   const ModuleLoadRequest* AsModuleRequest() const;