IndexedDB FS implementation

[firu11]

In addition to #4, I created a replacement billy.Filesystem implementation using IndexedDB as the underlying storage.

Why?

The Firefox OPFS implementation is extremely slow and nearly unusable on Linux. While it performs well for pure I/O, operations such as creating, moving, and deleting files/blobs are slow enough that Git’s internal operations can make commits take several seconds.

IndexedDB is a key-value database, so emulating a filesystem requires structuring the data across two stores:

• info - stores metadata about files and directories
  example:
  /CALENDAR_NAME/events/UUID.json -> name="UUID.json" size=116 mod_time=1777064650150 mode=438

• content - stores file contents (directories are not stored here)
  example:
  /CALENDAR_NAME/events/UUID.json -> binary data

This two-store-layout is inspired by hack-pad/go-indexeddb.

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Head-to-head comparison:

Device: Linux+Firefox laptop
Operation: Save Event (includes saving the event itself and git add+commit ops.)

OPFS: ~1.15s
IndexedDB: ~49ms