fix(language-core): generate codes for `:key` prop on `v-for` and `v-if` node

[so1ve]

I created a fake component to generate codes since IF_BRANCH nodes are not valid elements. Not sure if it is the best approach.

fixes #4539

[johnsoncodehk]

I temporarily changed the issue to a draft because we should wait for https://github.com/vuejs/core/issues/11322 to be fixed by the upstream. Forcibly handling unrecognized props in language tools will make the code we need to maintain too complex.

[johnsoncodehk]

Closed due to PR being outdated.