add to change low level backend

This is a replacement for #49

My test example:

class MemoryBackend {
  constructor() {
    this._store = new Map();
  }
  saveSuperblock(superblock) {
    this._store.set("!root", superblock)
  }
  loadSuperblock() {
    return this._store.get("!root")
  }
  readFile(inode) {
    return this._store.get(inode)
  }
  writeFile(inode, data) {
    return this._store.set(inode, data)
  }
  unlink(inode) {
    return this._store.delete(inode)
  }
  wipe() {
    this._store = new Map()
  }
  close() {

  }
}

var fs = new LightningFS("fs", { db: new MemoryBackend() });

(async () => {

    await fs.promises.writeFile('/foo', 'hello');
    const file = await fs.promises.readFile('/foo');
    console.log(file.toString());

})();

isomorphic-git / lightning-fs

add to change low level backend #91