Update rule selection to respect preview mode (#7195)
## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
Extends work in #7046 (some relevant discussion there)
Changes:
- All nursery rules are now referred to as preview rules
- Documentation for the nursery is updated to describe preview
- Adds a "PREVIEW" selector for preview rules
- This is primarily to allow `--preview --ignore PREVIEW --extend-select
FOO001,BAR200`
- Using `--preview` enables preview rules that match selectors
Notable decisions:
- Preview rules are not selectable by their rule code without enabling
preview
- Retains the "NURSERY" selector for backwards compatibility
- Nursery rules are selectable by their rule code for backwards
compatiblity
Additional work:
- Selection of preview rules without the "--preview" flag should display
a warning
- Use of deprecated nursery selection behavior should display a warning
- Nursery selection should be removed after some time
## Test Plan
<!-- How was it tested? -->
Manual confirmation (i.e. we don't have an preview rules yet just
nursery rules so I added a preview rule for manual testing)
New unit tests
---------
Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>
