go/types, x/tools: help tool authors support the new go/types.Alias node

[findleyr]

Following up on #46477 (comment), this umbrella issue tracks additional actions we can take to help tool authors support the new go/types.Alias node. We've learned a lot from our work on #64581, and should share this knowledge:

• Propose an x/exp/aliases transitional package based on x/tools/internal/aliases. We did something similar for generics in x/exp/typeparams: a new module with a transitional API for tools #50447.
• Write a guide for updating alias support. Again, we did something similar for generics.
• In the Go 1.23 release notes, ensure that the change to the gotypesalias GODEBUG flag is documented with instructions for tool authors to set gotypesalias=0.
• Reach out to authors of popular open source tools that may be affected.

(anyone is welcome to suggest more actions we can take)

CC @adonovan @timothy-king