feat: Add `_copier_conf.operation` variable

[lkubb]

Adds operation to _copier_conf, representing the current operation - either copy, recopy or update. This was proposed here: https://github.com/copier-org/copier/issues/1718#issuecomment-2282643624

I hope the way it's implemented and tested is as intended by @yajo. ~~The tests are quite synthetic,~~ I would expect the usual application to be _copier_conf.operation (!/=)= "update".

Fixes: https://github.com/copier-org/copier/issues/1725

Alternative to https://github.com/copier-org/copier/pull/1732