This change adds the new `activations` attribute to `Doc`. This
attribute can be used by trainable pipes to store their activations,
probabilities, and guesses for downstream users.

As an example, this change modifies the `tagger` and `senter` pipes to
add an `store_activations` option. When this option is enabled, the
probabilities and guesses are stored in `set_annotations`.

When the value is:

- A bool: all activations are stored when set to `True`.
- A List[str]: the activations named in the list are stored

So that they, like the other tests, also check that we get no
activations if no activations were requested.

Between the master and v4 branches.

Setters that take a different type than what the getter returns are still
problematic for MyPy. Replace the setter by a method, so that type inference
works everywhere.

This reverts commit 6e7b958.

Co-authored-by: Sofie Van Landeghem <svlandeg@users.noreply.github.com>

This means that we can also bring back `store_activations` setter.

We do not need to enumerate the activations anymore since `store_activations` is
`bool`.

This error was used when activations were still `Union[bool, List[str]]`.

- "probs" -> "probabilities"
- "guesses" -> "label_ids", except in the edit tree lemmatizer, where
  "guesses" -> "tree_ids".

This warning was used when we still allowed the user to specify
which activations to save.

Co-authored-by: Sofie Van Landeghem <svlandeg@users.noreply.github.com>

This reverts commit 0bd5730.