Use enumer to generate enum helpers

[imiric]

As mentioned in grafana/xk6-browser#65 (comment), instead of maintaining our own conversion maps and JSON marshalling implementations, we should use the enumer tool to do that for us.

We should add go:generate comments so that running go generate ./... will pick it up. See how this is done in k6:

k6/lib/netext/httpext/response.go

Line 33 in dae7000

//go:generate enumer -type=ResponseType -transform=snake -json -text -trimprefix ResponseType -output response_type_gen.go

.