Make buffer search aware of search direction (#24974)

This solves a couple of issues with Vim search by making the search
buffer and `SearchableItem` aware of the direction of the search. If
`SearchOptions::BACKWARDS` is set, all operations will be reversed. By
making `SearchableItem` aware of the direction, the correct active match
can be selected when searching backward.

Fixes #22506. This PR does not fix the last problem in that issue, but
that one is also tracked in #8049.

Release Notes:

- Fixes incorrect behavior of backward search in Vim mode

crates/search/src/search.rs

@@ -47,6 +47,8 @@ bitflags! {
         const CASE_SENSITIVE = 0b010;
         const INCLUDE_IGNORED = 0b100;
         const REGEX = 0b1000;
+        /// If set, reverse direction when finding the active match
+        const BACKWARDS = 0b10000;
     }
 }