feat: questionnaire validation

I’d love a possibility to validate questionnaire inputs. Being able to validate str against regex would already be a huge help, e.g. by specifying a field appropriately in copier.yml:

project_name:
  type: str
  help: Your project name
  validate: ^[a-zA-Z][_a-zA-Z0-9]+$

[barrywhart]

@yajo, @pawamoy: I have a possible fix for the test failures in #785. See PR #804.

[yajo]

Correct!

Another example:

your_age:
  type: int
  validator: "{% if your_age <= 0 %}Must be positive{% endif %}"