enable opened file to be saved without re-export

[TaufiqHdyt]

reference

inspiration from excalidraw (only for chrome)

handle download for non chromium based browser

[rusminto]

did you mean like how ms word automatically store the opened file? for browser app, i don't know if it good idea.
and showSaveFilePicker() is not supported yet on some browsers:

if the goal is to make the content persisted after tab refreshed, then maybe we can use localStorage.