fix(reactivity): remove Symbol.observable

[cexbrayat]

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.
Otherwise here is the compilation error:

ERROR in /Users/ced-pro/Code/ninjasquad/.../node_modules/@vue/reactivity/dist/reactivity.d.ts(134,13):
134:13 Property 'observable' does not exist on type 'SymbolConstructor'.
    132 |     [Symbol.observable]: infer V;
    133 | } ? {
  > 134 |     [Symbol.observable]: V;
        |             ^
    135 | } : {}) & (T extends {
    136 |     [Symbol.replace]: infer V;
    137 | } ? {

On top of #967

The bump to @types/node@v13 can be removed if you want: it's mainly here to reproduce the issue.

[pikax]

Can you provide a repo to reproduce this?

I tested with node v10.13.0, but was able to build without any issues

[cexbrayat]

It builds with @types/node up until v12, but it doesn't with v13. But the main issue is that an app using vue alpha.13 now has to add node in its tsconfig.json otherwise it does not compile.

[yyx990803]

We should remove it anyway since it's only Stage 1.