macOS: Selection for Find feature

Adds the `selection_for_search` action, with Cmd+E keybind by default.
This action inputs the currently selected text into the search
field without changing focus, matching standard macOS behavior.

src/input/Binding.zig

@@ -368,6 +368,9 @@ pub const Action = union(enum) {
     /// If a previous search is active, it is replaced.
     search: []const u8,
 
+    /// Input the selected text into the search field.
+    selection_for_search,
+
     /// Navigate the search results. If there is no active search, this
     /// is not performed.
     navigate_search: NavigateSearch,
@@ -1284,6 +1287,7 @@ pub const Action = union(enum) {
             .cursor_key,
             .search,
             .navigate_search,
+            .selection_for_search,
             .start_search,
             .end_search,
             .reset,