Add macOS trackpad gesture support (#1007)

Adds trackpad gesture support to canvas interaction.  Supports:

- Pinch to zoom
- Two-finger panning

Feature is off by default. Also by default, the browser user-agent must
include "Mac" (even if the feature has been enabled).

```ts
// Enable mac trackpad gestures
LiteGraph.macTrackpadGestures = true

// Disable the default Mac user-agent check
LiteGraph.macGesturesRequireMac = false
```

test/__snapshots__/litegraph.test.ts.snap

@@ -158,6 +158,8 @@ LiteGraphGlobal {
   "do_add_triggers_slots": false,
   "highlight_selected_group": true,
   "isInsideRectangle": [Function],
+  "macGesturesRequireMac": true,
+  "macTrackpadGestures": false,
   "middle_click_slot_add_default_node": false,
   "node_box_coloured_by_mode": false,
   "node_box_coloured_when_on": false,