[CT-2469] Quick UX improvements to model versions

[jtcohen6]

Discussed in: #6736 (comment)

Three quick hits that allow us to tell a more compelling story around model versions in the first cut (v1.5):

• By default, allow defining the latest_version of dim_customers in a file named dim_customers.sql (no suffix). The convention of dim_customers_vX.sql would also be supported — so this isn't actually a behavior change.
• Provide users with a mechanism (probably post-hook macro) to create an un-suffixed alias pointing to the latest model version, until we support this natively: [CT-2468] [Feature] For versioned models, automatically create view/clone of latest version in unsuffixed database location #7442
• Info-level log if you have an unpinned ref to a model that has a newer-than-latest (i.e. prerelease) version defined: "Heads up, your unpinned ref will roll over to the new version as soon as it's the latest."