Type safety for "loadFlow" function when using the extension

`loadFlow` receives a string that isn't type-safe, leading to possible annoying errors such as typos or `loadFlow` not failing when .flyde files were moved around (#112 ?)

Also, adding type safety to the inputs and outputs of a flow is very valuable.

For any future contributor who wants to take a look:
There's already some attempt I've made to do this - see https://github.com/flydelabs/flyde/blob/main/dev-server/src/service/generate-typings.ts (`dev-server` is the module used by the vs-code extension)