Keyboard: eat key before running mapping

To see why this is needed, compare the following flows:
  - user: press key 'j' with Super already pressed
  - Keyboard: handle mapping, focusing next view
  - Seat: send wl_keyboard.enter with keys Super and 'j'
  - Keyboard: eat key 'j'
versus:
  - user: press key 'j' with Super already pressed
  - Keyboard: eat key 'j'
  - Keyboard: handle mapping, focusing next view
  - Seat: send wl_keyboard.enter with key Super.

The necessity of this was already mentioned in 1e3b8ed1; however,
without a comment in code, it was removed in 393bfb42 as superfluous.
Hopefully, the newly added comment will prevent such mistakes in the
future.

Fixes https://github.com/riverwm/river/issues/978

river/Seat.zig

@@ -359,6 +359,25 @@ pub fn enterMode(self: *Self, mode_id: u32) void {
     }
 }
 
+/// Is there a user-defined mapping for passed keycode, modifiers and keyboard state?
+pub fn hasMapping(
+    self: *Self,
+    keycode: xkb.Keycode,
+    modifiers: wlr.Keyboard.ModifierMask,
+    released: bool,
+    xkb_state: *xkb.State,
+) bool {
+    const modes = &server.config.modes;
+    for (modes.items[self.mode_id].mappings.items) |*mapping| {
+        if (mapping.match(keycode, modifiers, released, xkb_state, .no_translate) or
+            mapping.match(keycode, modifiers, released, xkb_state, .translate))
+        {
+            return true;
+        }
+    }
+    return false;
+}
+
 /// Handle any user-defined mapping for passed keycode, modifiers and keyboard state
 /// Returns true if a mapping was run
 pub fn handleMapping(