feat: add warn function

[posva]

the warn function in Vue 3 adds a stacktrace so I think it's worth for library authors to be able to use it. However, it doesn't exist in Vue 2 (there is Vue.util.warn, but it doesn't have a stacktrace)

Because of this I thought of adding warn to vue-demi, but maybe a better place would be the composition-api directly 🤔 @pikax

[antfu]

I agree it's better to have this in the plugin directly, but would console.warn enough for printing stacktrace? See vue-next doing a bit more complex

https://github.com/vuejs/vue-next/blob/eb2ae44d94fe71f93cbabc5e049f03f422db48b0/packages/runtime-core/src/warning.ts#L33-L72

[posva]

We don't have the stacktrace info available publicly in vue@2 and I wonder if it can be added

[pikax]

I agree with this being added to the composition-api plugin

[posva]

Ok! I will move it to composition api next week then

[posva]

PR at vuejs/composition-api#596