Determine if we want to include default interaction & what to include

[kanitw]

As discussed briefly in #1657

@domoritz raises a very important point.

The problem is that I don't know what channel we would use (e.g. for highlighting) and how we resolve conflicts if the user uses that channel to encode data.

[kanitw]

In Vega-Lite, maybe it might be too much to always assume default interactions.

Similar to how we allow Polestar / Voyager 2's unit spec to overlay line/area with point as markers,
we can add a mode that automatically add interactions when this flag.
In this mode, we can make stronger assumption such as that users won't map fields to opacity and use opacity to encode selected (that's what Tableau does).

That said, for multi-views, think might gets trickier with multi-views that renders different data sources. (In Tableau, it seems like they don't link views by default in this case.)

(Since all of these "normalizer" is done before we construct Models in the compiler, it can be decoupled as another wrapper library / plugins. That's another conversation.)

[g3o2]

It probably depends on what kinds of interaction would be set as default and for which type of displays. Tooltips or markers are no problem, for they do not fundamentally change the display. However, any interaction that adds, rescales or filters marks is likely to be more difficult for the user to understand and for the developer to predict.

[kanitw]

Notes from a discussion last week:

• The problem with default interaction is that it's hard to override.
• At the same time, it can be really useful. We were considering a special config.interactive flag that augment a static spec with a set of default interactions (if the config.interactive = true). This way users can add default interaction if desired and if they do not, they do not have to worry about overriding this and that.