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/compiler-sfc/src/templateTransformAssetUrl.ts

@@ -69,8 +69,8 @@ export const transformAssetUrl: NodeTransform = (
 }
 
 function getImportsExpressionExp(
-  path: string | undefined,
-  hash: string | undefined,
+  path: string | null,
+  hash: string | null,
   loc: SourceLocation,
   context: TransformContext
 ): ExpressionNode {