feat(instanceWatch): watch nested value by path

[dsseng]

Reason: backwards compatibility with v2 (https://vuejs.org/v2/api/#vm-watch)

[dsseng]

Also, I think getValueByPath function might be helpful so we can extract it into shared package

[yyx990803]

This is going to be dropped in v3 (will only be supported in 2.x compat build). It's one of those inconsistent designs (only support dot but not bracket paths / complex exps)