Only treat RegExp instances as regular expressions

This affects matchReplaceBody and admin server CORS configuration. If
you previously passed something very regex-like but not actually a regex
(this is a very niche scenario) then it was handled as a regex. Now we
only actually use real regex instances, simplifying the code & logic.

Author: pimterry

src/admin/admin-server.ts

@@ -88,7 +88,7 @@ async function strictOriginMatch(
         return expectedOrigin === origin;
     }
 
-    if (_.isRegExp(expectedOrigin)) {
+    if (expectedOrigin instanceof RegExp) {
         return !!origin.match(expectedOrigin);
     }
 

src/rules/requests/request-step-definitions.ts

@@ -951,7 +951,7 @@ export class PassThroughStep extends Serializable implements RequestStepDefiniti
                 matchReplaceBody: !!this.transformRequest?.matchReplaceBody
                     ? this.transformRequest.matchReplaceBody.map(([match, result]) =>
                         [
-                            _.isRegExp(match)
+                            match instanceof RegExp
                                 ? { regexSource: match.source, flags: match.flags }
                                 : match,
                             result
@@ -981,7 +981,7 @@ export class PassThroughStep extends Serializable implements RequestStepDefiniti
                 matchReplaceBody: !!this.transformResponse?.matchReplaceBody
                     ? this.transformResponse.matchReplaceBody.map(([match, result]) =>
                         [
-                            _.isRegExp(match)
+                            match instanceof RegExp
                                 ? { regexSource: match.source, flags: match.flags }
                                 : match,
                             result