* 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.
This enables default support for parsing bigInt, optional chaining
and nullish coalescing, and also adds the `expressionPlugins`
compiler option for enabling additional parsing plugins listed at
https://babeljs.io/docs/en/next/babel-parser#plugins.
