Support multiple views

code for reference

```
# Note: it would be nicer to have only VariableLengthTuple, which would ensure the
# JS side always receives a list of strings and never a single string, but
# VariableLengthTuple won't coerce a single object into a tuple of length 1.
views = t.Union(
[
t.Instance(BaseView),
VariableLengthTuple(t.Instance(BaseView)),
],
).tag(
sync=True,
**ipywidgets.widget_serialization,
)
```

Start by locating the views definition shown in the issue and inspect BaseView and VariableLengthTuple, including how ipywidgets serialization reaches the JS side. Done means the feature accepts multiple views while preserving the expected serialized representation for a single view and a collection of views.

Written by the indexing model from the issue text.