Add Error.isError() feature (#2711)

Author: ddbeck

features/is-error.yml

@@ -0,0 +1,5 @@
+name: Error.isError()
+description: The `Error.isError()` static method checks whether a value is an `Error` object.
+spec: https://tc39.es/proposal-is-error/
+compat_features:
+  - javascript.builtins.Error.isError

features/is-error.yml.dist

@@ -0,0 +1,8 @@
+# Generated from: is-error.yml
+# Do not edit this file by hand. Edit the source file instead!
+
+status:
+  baseline: false
+  support: {}
+compat_features:
+  - javascript.builtins.Error.isError