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>

crates/ruff_macros/src/rule_code_prefix.rs

@@ -12,22 +12,14 @@ pub(crate) fn expand<'a>(
     let mut prefix_to_codes: BTreeMap<String, BTreeSet<String>> = BTreeMap::default();
     let mut code_to_attributes: BTreeMap<String, &[Attribute]> = BTreeMap::default();
 
-    for (variant, group, attr) in variants {
+    for (variant, .., attr) in variants {
         let code_str = variant.to_string();
-        // Nursery rules have to be explicitly selected, so we ignore them when looking at prefixes.
-        if is_nursery(group) {
+        for i in 1..=code_str.len() {
+            let prefix = code_str[..i].to_string();
             prefix_to_codes
-                .entry(code_str.clone())
+                .entry(prefix)
                 .or_default()
                 .insert(code_str.clone());
-        } else {
-            for i in 1..=code_str.len() {
-                let prefix = code_str[..i].to_string();
-                prefix_to_codes
-                    .entry(prefix)
-                    .or_default()
-                    .insert(code_str.clone());
-            }
         }
 
         code_to_attributes.insert(code_str, attr);
@@ -125,14 +117,3 @@ pub(crate) fn get_prefix_ident(prefix: &str) -> Ident {
     };
     Ident::new(&prefix, Span::call_site())
 }
-
-/// Returns true if the given group is the "nursery" group.
-pub(crate) fn is_nursery(group: &Path) -> bool {
-    let group = group
-        .segments
-        .iter()
-        .map(|segment| segment.ident.to_string())
-        .collect::<Vec<String>>()
-        .join("::");
-    group == "RuleGroup::Nursery"
-}