fix(reactivity): remove Symbol.observable (#968)

* chore: add @types/node as an explicit dependency

As `tsconfig.json` references it, it should be listed in the dependencies.
It currently uses an older version: this commit also bumps to the latest v12 version, and fixes a typing issue.

* fix(reactivity): remove Symbol.observable

`Symbol.observable` is brought by `@types/node@12` and is not a "well-known" typescript symbol https://www.typescriptlang.org/docs/handbook/symbols.html that can be find in lib.es20xx like the others. It has been removed in `@types/node@v13`.
It means that an application using vue@3.0.0-alpha.13 does not compile unless it explicitely adds `@types/node@v12` as a dependency and `node` in its own tsconfig types.

packages/reactivity/src/ref.ts

@@ -151,9 +151,6 @@ type SymbolExtract<T> = (T extends { [Symbol.asyncIterator]: infer V }
   (T extends { [Symbol.iterator]: infer V } ? { [Symbol.iterator]: V } : {}) &
   (T extends { [Symbol.match]: infer V } ? { [Symbol.match]: V } : {}) &
   (T extends { [Symbol.matchAll]: infer V } ? { [Symbol.matchAll]: V } : {}) &
-  (T extends { [Symbol.observable]: infer V }
-    ? { [Symbol.observable]: V }
-    : {}) &
   (T extends { [Symbol.replace]: infer V } ? { [Symbol.replace]: V } : {}) &
   (T extends { [Symbol.search]: infer V } ? { [Symbol.search]: V } : {}) &
   (T extends { [Symbol.species]: infer V } ? { [Symbol.species]: V } : {}) &