fix: add event modifiers for mpl 3.7

Author: ianhi

src/mpl_widget.ts

@@ -757,6 +757,7 @@ export class MPLCanvasView extends DOMWidgetView {
                 y: y,
                 button: event.button,
                 step: event.step,
+                modifiers: utils.getModifiers(event),
                 guiEvent: utils.get_simple_keys(event),
             });
         };

src/utils.ts

@@ -33,3 +33,20 @@ export function getContext(canvas: HTMLCanvasElement) {
     }
     return context;
 }
+
+export function getModifiers(event: MouseEvent) {
+    const mods = [];
+    if (event.ctrlKey) {
+        mods.push('ctrl');
+    }
+    if (event.altKey) {
+        mods.push('alt');
+    }
+    if (event.shiftKey) {
+        mods.push('shift');
+    }
+    if (event.metaKey) {
+        mods.push('meta');
+    }
+    return mods;
+}