Attachment management

[zefhemel]

As of #71 attachments are implemented. You can copy & past, or drag & drop files into a page, and they will be uploaded and linked. There are a few things missing:

• Delete attachments (inside of SB, you can just delete files outside of SB as you like, of course)
• Attachment indexing (tracking which page reference what attachment). Currently all attachments are linked either via [text](attachment/PATH) or ![text](attachment/PATH) so indexing based on this should work. Indexing would allow e.g. renaming of attachments or give warnings (or remove references) when to-be-deleted-attachments are about to be deleted.

[zefhemel]

Syscalls for managing all these things are available (although untested): https://github.com/silverbulletmd/silverbullet/blob/main/packages/plugos-silverbullet-syscall/space.ts#L26