Add transformstream-transformer-cancel feature

Author: MattiasBuelens

features/transformstream-transformer-cancel.yml

@@ -0,0 +1,4 @@
+name: "TransformStream: transformer.cancel method"
+description: The `cancel` method of a `TransformStream` transformer allows you to clean up resources when the readable side is cancelled or the writable side is aborted.
+spec: https://streams.spec.whatwg.org/#dom-transformer-cancel
+group: streams

features/transformstream-transformer-cancel.yml.dist

@@ -0,0 +1,6 @@
+# Generated from: transformstream-transformer-cancel.yml
+# Do not edit this file by hand. Edit the source file instead!
+
+status:
+  baseline: false
+  support: {}