Enable the "Subscribe to Label" github action

This allows users to follow certain labels and automatically get @-mentioned
when they are applied to an issue or a pull request.

See https://github.com/bytecodealliance/subscribe-to-label-action for details.

Fixes #1234

.github/subscribe-to-label.json

@@ -0,0 +1,3 @@
+{
+  "peterhuene": ["wasmtime:api", "wasmtime:c-api"]
+}