Declare Heroku-20 stack support (#295)

Similar to projectriff/streaming-http-adapter-buildpack#13, we would like to be able to include `node-function-buildpack` in our `heroku/buildpacks:20` builder.

I would love to try this out to make sure it doesn't just immediately fail, but the README seems to be out of date on producing a locally testable buildpack. It reference `make`, but there is no `Makefile`. I tried `scripts/build.sh`, but that only builds the binaries. If you were to create a draft release, would that trigger your GitHub actions to publish a reference-able image to do some smoke tests? I don't expect any problems, but I would feel better knowing I at least packaged a builder with the artifact.

buildpack.toml

@@ -20,6 +20,9 @@ name     = "riff NodeJS Function Buildpack"
 version  = "{{.version}}"
 homepage = "https://github.com/projectriff/node-function-buildpack"
 
+[[stacks]]
+id = "heroku-20"
+
 [[stacks]]
 id = "heroku-18"
 
@@ -45,7 +48,7 @@ name    = "riff NodeJS Invoker"
 version = "0.3.0"
 uri     = "https://storage.googleapis.com/download/storage/v1/b/projectriff/o/node-function-invoker%2Freleases%2Fv0.3.0%2Fnode-function-invoker-0.3.0.tgz?generation=1589400159284258&alt=media"
 sha256  = "83d4f64137aed4199c97031856ddcddf46cb2e173d805b46240d54d036d83241"
-stacks  = [ "heroku-18", "io.buildpacks.stacks.bionic", "org.cloudfoundry.stacks.cflinuxfs3" ]
+stacks  = [ "heroku-18", "heroku-20", "io.buildpacks.stacks.bionic", "org.cloudfoundry.stacks.cflinuxfs3" ]
 
   [[metadata.dependencies.licenses]]
   type = "Apache-2.0"