Prefer snapshot to resolver, while maintaining them as synoynms

[mpilgrem]

Building on #4256:

Capable of being included in Stack 2.15.1, because they do not interfere with users' current practices:

• Introduce --snapshot as a synonym for --resolver
• Introduce config set snapshot command, corresponding to config set resolver
• Prefer the snapshot: key to resolver: in Stack's online examples and in-app messages
• Prefer --snapshot to --resolver in Stack's online examples and in-app messages
• Put 'snapshot' ahead of 'resolver' in Stack's online documentation where synonyms introduced
• Refer to 'snapshot' or 'snapshot resolver' in in-app messages where 'resolver' is currently used.
• Use snapshot: in Stack's tests
• Use snapshot: in Stack's own project-level configuration files

Better delayed to post-Stack 2.15.1 and its adoption by GitHub-hosted runners

• Prefer snapshot: to resolver: in stack init
• Update the instance ToJSON Project to use snapshot: for the resolver field
• Possibly, rename the resolver field of the Package type snapshot