transitionType: allow passing extra warning text

[MattSturgeon]

See #2194 (reply in thread) for original discussion.

We should refactor transitionType to allow passing in additional warning text, such as migration instructions.

Similar to mkRemovedOptionModule, and in contrast to mkRenamedOptionModule, there often isn't enough context in the default warning for users to know how to migrate. This is especially true when the old and new type are similar, as is the case for lualine's refactor (#2080).

I think once this is done, we should consider attempting to upstream a transition type to nixpkgs so that a) it can be used outside of nixvim and b) it can share code with coercedTo.