PoC: File/dir picker obscured by PiP

Read env vars, read/write files

PoC works with showOpenFilePicker(), showSaveFilePicker(), showDirectoryPicker().

After patch for crbug.com/40076120, PiP obscuring is mitigated only for input type=file element (either by closing or tucking PiP window).

Variations:

• Read environment variables (Default)
• Read file at arbitrary location
• Write file at arbitrary location