Add weighted search

Fixes #74

lib/helpers/extensions/weighted_search.dart

@@ -0,0 +1,47 @@
+import "package:collection/collection.dart";
+import "package:fast_immutable_collections/fast_immutable_collections.dart";
+
+extension WeightedSearch<T> on Iterable<T> {
+  /// The items with a field matching [query], best match first.
+  IList<T> search(
+    String query,
+    Iterable<(String?, int)> Function(T item) fieldsOf,
+  ) {
+    final needle = query.trim().toLowerCase();
+    if (needle.isEmpty) return toIList();
+
+    int qualityOf(String text) {
+      final haystack = text.toLowerCase();
+      if (haystack == needle) return 4;
+      if (haystack.startsWith(needle)) return 3;
+      if (haystack
+          .split(RegExp(r"[\s_\-:/.@#!]+"))
+          .any((w) => w.startsWith(needle))) {
+        return 2;
+      }
+      if (haystack.contains(needle)) return 1;
+      return 0;
+    }
+
+    return .new(
+      map(
+            (item) => (
+              item,
+              fieldsOf(item)
+                      .map(
+                        (field) => switch (field) {
+                          (final text?, final weight) =>
+                            qualityOf(text) * weight,
+                          (null, _) => 0,
+                        },
+                      )
+                      .maxOrNull ??
+                  0,
+            ),
+          )
+          .where((scored) => scored.$2 > 0)
+          .sortedBy((scored) => -scored.$2)
+          .map((scored) => scored.$1),
+    );
+  }
+}