Add `--mock` flag support

[softmarshmallow]

Component variant on design tool often contains state of user interaction - e.g. A input cursor or a help text / actual input, a hover state.

See, the hover state is not a state that is exposed to outer component interface, it's a interaction state provided by native callbacks. So on design tool, hover=true variant should be marked as --mock so that design-sdk won't accidently recognize it as a state that is controllable by parent accessors.