Saving, updating, and reading files from disk

[Keavon]

https://developer.mozilla.org/en-US/docs/Web/API/File_System_Access_API https://developer.chrome.com/articles/file-system-access/ https://developer.chrome.com/articles/file-handling/

Supported on Chromium and Firefox, but Safari seems to not support writing (or it doesn't support the async version of the API, but it does support the sync WebWorker-only version or something?). So we'll potentially have to tailor the feature experience depending on the browser.

Eventual desired behavior: the document gets auto-saved in real time as the user works (perhaps whenever the history state is updated) to the browser's IndexedDB. If the user hits CtrlS/File > Save, it saves the file to disk at the existing document location, if that is available. If not, it asks the user to grant permission for a specific folder. We may need to build a file browser UI in that case to let the user pick where to organize the file within the granted folder scope. Unsupported browsers should potentially fall back to the current download-on-save/browse-on-open approach.