Make tool to export localStorage files

[curiousdannii]

https://intfiction.org/t/where-does-parchment-save-the-transcript/56473

It would be helpful to have a standalone tool to export localStorage files. Maybe skipping autosaves?

[curiousdannii]

Extra improvements:

• [x] check for missing filetype
• [x] use actual modified timestamp
• [x] actually enable compression