BREAKING CHANGE: replae `watch(fn, options?)` with `watchEffect`

    The `watch(fn, options?)` signature has been replaced by the new
    `watchEffect` API, which has the same usage and behavior. `watch`
    now only supports the `watch(source, cb, options?)` signautre.

BREAKING CHANGE: reactive arrays no longer unwraps contained refs

    When reactive arrays contain refs, especially a mix of refs and
    plain values, Array prototype methods will fail to function
    properly - e.g. sort() or reverse() will overwrite the ref's value
    instead of moving it (see #737).

    Ensuring correct behavior for all possible Array methods while
    retaining the ref unwrapping behavior is exceedinly complicated; In
    addition, even if Vue handles the built-in methods internally, it
    would still break when the user attempts to use a 3rd party utility
    functioon (e.g. lodash) on a reactive array containing refs.

    After this commit, similar to other collection types like Map and
    Set, Arrays will no longer automatically unwrap contained refs.

    The usage of mixed refs and plain values in Arrays should be rare in
    practice. In cases where this is necessary, the user can create a
    computed property that performs the unwrapping.

perf(reactivity): only trigger all effects on Array length mutation if new length is shorter than old length

Bumps [typescript](https://github.com/Microsoft/TypeScript) from 3.7.5 to 3.8.2.
- [Release notes](https://github.com/Microsoft/TypeScript/releases)
- [Commits](https://github.com/Microsoft/TypeScript/commits

)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

close #738

This makes it possible to create a separate package that uses runtime compilation.

fix #740

Bumps [postcss](https://github.com/postcss/postcss) from 7.0.26 to 7.0.27.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/master/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/7.0.26...7.0.27

)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

fix #735

fix #709

fix #733

BREAKING CHANGE: `watch` behavior has been adjusted.

    - When using the `watch(source, callback, options?)` signature, the
      callback now fires lazily by default (consistent with 2.x
      behavior).

      Note that the `watch(effect, options?)` signature is still eager,
      since it must invoke the `effect` immediately to collect
      dependencies.

    - The `lazy` option has been replaced by the opposite `immediate`
      option, which defaults to `false`. (It's ignored when using the
      effect signature)

    - Due to the above changes, the `watch` option in Options API now
      behaves exactly the same as 2.x.

    - When using the effect signature or `{ immediate: true }`, the
      intital execution is now performed synchronously instead of
      deferred until the component is mounted. This is necessary for
      certain use cases to work properly with `async setup()` and
      Suspense.

      The side effect of this is the immediate watcher invocation will
      no longer have access to the mounted DOM. However, the watcher can
      be initiated inside `onMounted` to retain previous behavior.

fix #730