chore: define enums for mouse buttons and their states

lib/src/core/mouse/button.dart

@@ -0,0 +1,45 @@
+enum TerminalMouseButton {
+  left,
+
+  middle,
+
+  right,
+
+  wheelUp,
+
+  wheelDown,
+
+  wheelLeft,
+
+  wheelRight,
+}
+
+extension ButtonIDExtension on TerminalMouseButton {
+  /// The id that is used to report a button press or release to the terminal.
+  int get id {
+    switch (this) {
+      case TerminalMouseButton.left:
+      case TerminalMouseButton.middle:
+      case TerminalMouseButton.right:
+        return index;
+      // Mouse wheel up / down use button IDs 4 = 0100 (binary) and
+      // 5 = 0101 (binary). The bits three and four of the button
+      // are transposed by 64 and 128 respectively, when reporting the id of
+      // the button and have have to be adjusted correspondingly.
+      case TerminalMouseButton.wheelUp:
+        return 64 + 4;
+      case TerminalMouseButton.wheelDown:
+        return 64 + 5;
+      case TerminalMouseButton.wheelLeft:
+        return 64 + 6;
+      case TerminalMouseButton.wheelRight:
+        return 64 + 7;
+    }
+  }
+
+  /// True if the button belongs to a wheel.
+  bool get isWheel {
+    return TerminalMouseButton.wheelUp.index <= index &&
+        index <= TerminalMouseButton.wheelRight.index;
+  }
+}

lib/src/core/mouse/button_state.dart

@@ -0,0 +1,5 @@
+enum TerminalMouseButtonState {
+  up,
+
+  down,
+}