Validate `with_entrypoint` and `with_transitions` to check if the types are strings

[elijahbenizzy]

This should fail for two reasons and mention that the type should be a string, not an action. It should also explain why (because they don't have names until they're added in):

from burr.core import action, State, ApplicationBuilder, default


@action(reads=[], writes=["list"])
def append(state: State) -> tuple[dict, State]:
    return state.append(list=1)


app = (
    ApplicationBuilder()
    .with_state(list=[])
    .with_actions(append=append)
    .with_entrypoint(append)
    .with_transitions(
        (append, append, default),
    )
    .build()
)

old_state = app.state
action, result, new_state = app.step()

print(old_state, new_state)