fix(runtime-core): InjectionKey compatible with symbol type

[cexbrayat]

The current type of InjectionKey does not allow to use it as an ES computed property.
This is problematic as the API of mount in VTU lets you add providers with provide: { key: value }.
If you want to use a key defined as:

export const storeKey: InjectionKey<Store<State>> = Symbol();

Then you currently have to cast the key as symbol:

const wrapper = mount(App, { global: { provide: { [storeKey as symbol]: store } } });

This commit allows to use:

const wrapper = mount(AppWithVuex, { global: { provide: { [storeKey]: store } } });

[SimonSimCity]

Seems this has been picked up again in #5089