Update on "[compiler] Flow support for playground"

Summary: The playground currently has limited support for Flow files--it tries to parse them if the // flow sigil is on the fist line, but this is often not the case for files one would like to inspect in practice. more importantly, component syntax isn't supported even then, because it depends on the Hermes parser.

This diff improves the state of flow support in the playground to make it more useful: when we see `flow` anywhere in the file, we'll assume it's a flow file, parse it with the Hermes parser, and disable typescript-specific features of Monaco editor.

[ghstack-poisoned]