When ctrl-clicking to select multiple cells, use cmd on mac

If we use ctrl with the click when selecting multiple cells, it shows
the context menu. Instead, we use the Command key (meta) when we have
detected we're on a mac.

lib/importer/assets/js/selectable_table.js

@@ -1,4 +1,18 @@
+const get_platform = () =>  {
+  // userAgentData is not widely supported yet
+  if (typeof navigator.userAgentData !== 'undefined' && navigator.userAgentData != null) {
+      return navigator.userAgentData.platform;
+  }
+  // Currently deprecated
+  if (typeof navigator.platform !== 'undefined') {
+      return navigator.platform;
+  }
+  return 'unknown';
+}
+
 window.addEventListener("load", function() {
+    const isMac = /mac/i.test(get_platform())
+
     const CellSelectedClassName = "selected";
     const CellSelectedBottomClassName = "bottom";
     const CellSelectedTopClassName = "top";
@@ -833,6 +847,7 @@ window.addEventListener("load", function() {
         updateKey(keyEvent.shiftKey, "Shift");
         updateKey(keyEvent.ctrlKey, "Control");
         updateKey(keyEvent.altKey, "Alt");
+        updateKey(keyEvent.metaKey, "Meta");
       }
 
       DocumentSelectMode.addEvent("keydown", updateFromKeyEvent);
@@ -855,7 +870,9 @@ window.addEventListener("load", function() {
         // If we have control-dragged, add the new selection to the existing selection
         // Otherwise, replace the existing selection with the new one
         const mergeSelection = function(newSelection) {
-          if (depressedKeys.has("Control")) {
+          const selectionKey = isMac ? "Meta" : "Control"
+
+          if (depressedKeys.has(selectionKey)) {
             return initialSelection.includingSelection(newSelection);
           } else {
             return newSelection;