Add default translation plugin

[jakelishman]

At the moment, our available translation stages are called translator and synthesis, both of which are fine names. We default to translator unless overridden by a backend, or the user.

We may start to add more support for discrete-basis backend compilation over the course of Qiskit 2.x, or wish to modify the default method in the presence of fractional 2q gates, or so son. Adding a new default called default, which simply wraps translator (to begin with) gives us the semantic freedom to make some of those changes within the major version without a hard API break.