add start_search binding and apprt action

src/input/Binding.zig

@@ -340,6 +340,10 @@ pub const Action = union(enum) {
     /// is not performed.
     navigate_search: NavigateSearch,
 
+    /// Start a search if it isn't started already. This doesn't set any
+    /// search terms, but opens the UI for searching.
+    start_search,
+
     /// Clear the screen and all scrollback.
     clear_screen,
 
@@ -1167,6 +1171,7 @@ pub const Action = union(enum) {
             .cursor_key,
             .search,
             .navigate_search,
+            .start_search,
             .reset,
             .copy_to_clipboard,
             .copy_url_to_clipboard,

src/input/command.zig

@@ -163,6 +163,12 @@ fn actionCommands(action: Action.Key) []const Command {
             .description = "Paste the contents of the selection clipboard.",
         }},
 
+        .start_search => comptime &.{.{
+            .action = .start_search,
+            .title = "Start Search",
+            .description = "Start a search if one isn't already active.",
+        }},
+
         .navigate_search => comptime &.{ .{
             .action = .{ .navigate_search = .next },
             .title = "Next Search Result",