feat(deprecated-error-handling-warning): add console warning when code sets the flag to bad mode

It will console.warn and give the call site (via stack) of where it was set to use the deprecated error handling mode

Author: benlesh

src/internal/config.ts

@@ -1,3 +1,5 @@
+let _enable_super_gross_mode_that_will_cause_bad_things = false;
+
 /**
  * The global configuration object for RxJS, used to configure things
  * like what Promise contructor should used to create Promises
@@ -17,5 +19,17 @@ export const config = {
    * an unhandled error. DO NOT USE THIS FLAG UNLESS IT'S NEEDED TO BY TIME
    * FOR MIGRATION REASONS.
    */
-  useDeprecatedSynchronousErrorHandling: false,
+  set useDeprecatedSynchronousErrorHandling(value: boolean) {
+    if (value) {
+      const error = new Error();
+      console.warn('DEPRECATED! RxJS was set to use deprecated synchronous error handling behavior by code at: \n' + error.stack);
+    } else if (_enable_super_gross_mode_that_will_cause_bad_things) {
+      console.log('RxJS: Back to a better error behavior. Thank you. <3');
+    }
+    _enable_super_gross_mode_that_will_cause_bad_things = value;
+  },
+
+  get useDeprecatedSynchronousErrorHandling() {
+    return _enable_super_gross_mode_that_will_cause_bad_things;
+  }
 };