[cfe] Add ConstructorEncodingStrategy

This adds [ConstructorEncodingStrategy] similar to
[PropertyEncodingStrategy] which allows for separating
[ConstructorDeclaration] from [ConstructorEncoding] such that we don't
need special [ConstructorDeclaration]s for extension (types).

Change-Id: I1c7f17e1759d47abae4af97d84455a37cbb70c17
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/435460
Commit-Queue: Johnni Winther <[email protected]>
Reviewed-by: Chloe Stefantsova <[email protected]>

Author: johnniwinther