fix(runtime-core): align the merge strategies of component options with the behavior of vue2

[HcySunYang]

Fix: #3566, #2791

Still WIP, points that need to be optimized:

• improve the type
• more test cases

After the compat branch is merged, I need to re-work on this

[yyx990803]

Great work in this PR - I tried to rebase it locally while figuring out a better caching strategy. We don't want to run options merging for every instance since this was a major perf overhead in Vue 2, but in Vue 3 caching becomes a bit tricky due to different app instances may have different global mixins and merge strategies.

The conflicts got a bit out of hand so I decided to start from a clean branch instead.

• In 1e35a86 we are now caching merged options on the app context.

  • Note this removes the 3rd vm argument from merge strategy functions so that we can cache the merge result for all instances in the same app. This argument is technically unnecessary so I believe this is a worthwhile tradeoff.

• e2ca67b is largely based on this PR (especially the tests)

  • Removed the watch test case because users should not rely on the merge result of watch callbacks.

This also ensures the behavior becomes more consistent between the compat build and the non-compat build.