Add namespace to runtime shebang flags

[jsejcksn]

It might be a good idea to namespace the custom flags with something like --vercel- in order to avoid potential future collisions (however unlikely):

#!/usr/bin/env deno run --unstable --vercel-version v1.11.4 --vercel-include-files myjson.json

Aside: Is the = syntax supported here? e.g. --version=v1.11.4