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.

package.json

@@ -46,6 +46,7 @@
     "@rollup/plugin-node-resolve": "^7.1.1",
     "@rollup/plugin-replace": "^2.2.1",
     "@types/jest": "^25.1.4",
+    "@types/node": "13.11.1",
     "@types/puppeteer": "^2.0.0",
     "brotli": "^1.3.2",
     "chalk": "^2.4.2",