feat: add system mode to the theme selector (#7853)

Co-authored-by: dwelle <5153846+dwelle@users.noreply.github.com>

packages/excalidraw/locales/en.json

@@ -110,6 +110,7 @@
     "showStroke": "Show stroke color picker",
     "showBackground": "Show background color picker",
     "toggleTheme": "Toggle light/dark theme",
+    "theme": "Theme",
     "personalLib": "Personal Library",
     "excalidrawLib": "Excalidraw Library",
     "decreaseFontSize": "Decrease font size",
@@ -180,6 +181,7 @@
     "fullScreen": "Full screen",
     "darkMode": "Dark mode",
     "lightMode": "Light mode",
+    "systemMode": "System mode",
     "zenMode": "Zen mode",
     "objectsSnapMode": "Snap to objects",
     "exitZenMode": "Exit zen mode",