Expose the current phase in context

[noirbizarre]

Actual Situation

Extensions have no way of knowing the current phase (aka. questions, render, tasks or migration).

As a consequence, they have to guess which context variable is available and safe for them to use.

Ex: copier-templates-extensions was guessing the rendering phase with the presence of _copier_conf context var but is broken since 9.4 due to its heuristic not working anymore

Desired Situation

Jinja extension can selectively run during the phase they intend to, and they have a safe way of knowing which phase is currently being executed

Proposed solution

In addition to an _operation context variable which has already been proposed and pending merge (#1733), a _phase context variable is also added.
Its value can be one of questions (or prompt), render, migrate or tasks.