Making `redeclare` a language feature?

If we were to accept [the proposal to make override a language feature](https://github.com/dart-lang/language/blob/main/working/1610%20-%20override/proposal.md), would we also want to make the `redeclare` annotation a language feature? It serves a similar function, but for extension types.

Read the linked working/1610 - override/proposal.md proposal and compare its treatment of override with the redeclare annotation for extension types. The issue is asking for a language-design decision, so done means reaching and documenting a decision about whether redeclare should become a language feature.

Written by the indexing model from the issue text.