types(inject): allow inject key to be number

[yangmingshan]

#2616 add number type to provide key, so should inject key.

[posva]

This looks good but I wonder if it makes sense to allow numbers at all for keys with inject/provide

[yangmingshan]

It's necessary for https://github.com/vuejs/vue-next/pull/2616/files#diff-bef066f3036cebd9b6db8a01d40f6eb0fdfe71c06da414b7d226652c016d7efdR658, otherwise TS will report error.

[yangmingshan]

@posva Can we move this PR forward?

[posva]

I'm not sure we should allow injecting numerical keys because they are very likely to collision and hard to debug. So overall a bad practice that is better not to be supported.
Like you said, the number was added to inject to make it pass typings but I wonder if we shouldn't even remove that instead

[yangmingshan]

I agree, I'm closing this, and open #3196