fix(editor): Arrow control on firefox

I forgor to check which key was pressed.
Due to an edge case, in which Firefox required 2 key presses to move from math to text, pressing left twice causes the editor to think you want to go left.
However, this only affected Firefox users, so we'll just fix this quietly :)

src/math-editor/editor.js

@@ -188,13 +188,15 @@ class Editor {
                     if ( // At start (from right)
                         documentSelection.anchorNode.nodeName.toLowerCase() === "#text" &&
                         elementList[selectionIndex - 1]?.nodeName === "MATH" &&
-                        documentSelection.anchorOffset === 0
+                        documentSelection.anchorOffset === 0 &&
+                        event.code === "ArrowLeft"
                     ) {
                         focus(elementList[selectionIndex - 1])
                     } else if ( // At end (from left)
                         documentSelection.anchorNode.nodeName.toLowerCase() === "#text" &&
                         elementList[selectionIndex + 1]?.nodeName === "MATH" &&
-                        documentSelection.anchorOffset === documentSelection.anchorNode.textContent.length
+                        documentSelection.anchorOffset === documentSelection.anchorNode.textContent.length &&
+                        event.code === "ArrowRight"
                     ) {
                         focus(elementList[selectionIndex + 1])
                     }