chore: upgrade vue to 2.6.14

[ygj6]

This PR and vuejs/vue#12132 work together to solve the problem of compilation warnings after the vue in composition-api is upgraded to the latest version.

Before vuejs/vue#12132 is merged, we will see that ci fails to run.

[ygj6]

This PR and vuejs/vue#12132 work together to solve the problem of compilation warnings after the vue in composition-api is upgraded to the latest version.

Before vuejs/vue#12132 is merged, we will see that ci fails to run.

@antfu @posva Now it is solved by shielding the warning, no longer dependent on vuejs/vue#12132

[posva]

After further investigation, your initial solution (vuejs/vue#12132) is probably a better idea. Merging that one would allow to not change anything here, right?

[ygj6]

After further investigation, your initial solution (vuejs/vue#12132) is probably a better idea. Merging that one would allow to not change anything here, right?

Here I have deleted invalid module extension of VueConstructor:
https://github.com/vuejs/composition-api/pull/731/files#:~:text=interface%20VueConstructor%20%7B,defineReactive(

Because they are already defined in vue/types/vue, so the extension here will not take effect.

I noticed that you added the null type here：
dd2cd6b#:~:text=export%20function%20warn(msg%3A%20string%2C%20vm%3F%3A%20Vue%20%7C%20null)%20%7B

but there is no null type in Vue: https://github.com/vuejs/vue/blob/656a3cbda089c26938802707223254990631b91f/types/vue.d.ts#L125

which will cause a warning, so I temporarily deleted the null here: https://github.com/vuejs/composition-api/pull/731/files#:~:text=export%20function%20warn(msg%3A%20string%2C%20vm%3F%3A%20Vue%20%7C%20null)

If don't want to delete null, we also could add null type in Vue. @posva Which solution do you think is better?