Find a way to signal to users that they should never call "resolve" on tokens

[eladb]

Currently, Tokens implement a method resolve which is called to resolve the token during synthesis, but users are tempted to invoke this method in various situations (i.e. to "resolve" the value in runtime, to "resolve" the value so they can reason about it, etc).

We should hide this method and/or signal that it shouldn't be called by users.

I propose that:

1. We rename this method _resolve.
2. We make it private and use duck-typing to identify the token (public/private is just a typescript annotation, doesn't apply at runtime).

[rix0rrr]

Both methods hide it from jsii, so jsii users will never be able to implement a new subclass of Token. That seems not ideal.

[eladb]

That's a good point, but what is the use case for actually extending tokens by non-framework code? I think it is very limited and maybe even something we want to block, because it's extremely error prone. If there will be a clear use case, we can always provide a base class such as CustomToken that exposes a method to override.