WRITEME: fix problematic snippet picker

[Laren-AWS]

The snippet picking code in .tools/readmes/scanner.py that determines which snippet to link to in the README could be better. Currently, it picks the first snippet listed for scenarios, which isn't always the best choice and causes some pain for example writers.

Probable solution is to implement a way to tag a snippet in the metadata as the "primary" or "linkable" so that WRITEME can deterministically find it and use it.