Use phase from Copier 9.5 to execute context hook only when rendering files

[pawamoy]

Fixes #7. Needs copier-org/copier#1948.

[pawamoy]

After a lot of confusion, it turns out the key error originally mentioned in #7 is due to the fact that a new question was introduced, and the context in the "prompt" phase does not contain this question (and its answer) yet (or something like that 😵).

Instead of "fixing" this by preventing context hooks from running in other phases than "render", I suggest we simply document that the hook will run in all these different phases (and that the phase will be available in context["_copier_phase"] starting with Copier 9.5) and that extension authors should make sure to handle key errors, or can use conditionals based on the phase.

I note that we could add syntactic sugar (@noirbizarre 😉) in the form of parameter injection, allowing extension authors to accept phase: Phase or other kind of parameters in their context hook signature, so that these values are typed and easily available (instead of having to find them in the context dict). Alternatively, we could use a TypedDict to type the context, at least the parts that do not move (i.e. everything except answers IIUC).