Reactivity: add and expose a shallow function

[jods4]

What problem does this feature solve?

In addition to reactive, readonly and shallowReadonly, add the missing shallow method and export it.

If one desires to a have reactive object, whose contents are not reactive (as an optimization or for explicitness), one has to call markNonReactive on every value set on a reactive object.
This is ok when initializing the object, but becomes a lot of effort when it has to be put at any place where a property is set to a new value.

The code to implement shallow already exists, as createGetter and createSetter accept two arguments isReadonly and shallow (consistency: why not isShallow?). In fact it's the only combination of flags out of the four possible that is missing.

What does the proposed API look like?

shallow method, makes a reactive object like reactive, readonly and shallowReadonly but doesn't make values returned by get reactive.

[dsseng]

I'm working on this one now.

[jods4]

I'm really wishing for a shallowRef in Vue core as well. @yyx990803 is it possible?

Typical use case is to store immutable/non-reactive (big) arrays or (deep) objects (for perf). E.g. if you want to display a large list and plan on setting the ref to a new array to avoid watching all array indexes (ties in with your comments about my fast mutable array).

Of course, it's trivial to build one out of @vue/reactivity but I feel like it's fundamental enough to be part of core.

[yyx990803]

@jods fyi: e9024bf