Restructure frontend code to allow for extensions (#4721)

# Description of Changes
Move frontend code into `core` folder and add infrastructure for
`proprietary` folder to include premium, non-OSS features

frontend/src/core/constants/toolPanel.ts

@@ -0,0 +1,5 @@
+// Tool panel constants
+
+export type ToolPanelMode = 'sidebar' | 'fullscreen';
+
+export const DEFAULT_TOOL_PANEL_MODE: ToolPanelMode = 'sidebar';