Incorrect `IDB` interface

I noticed that current `IDB` interface had two errors:
1. The constructor type was typed as a regular function
2. The `readFile` was mis-typed as `loadFile`

`/lightning-fs/index.d.ts`
```diff
export interface IDB {
- constructor(dbname: string, storename: string): IDB
+ new (dbname: string, storename: string): IDB
saveSuperblock(sb: Uint8Array): TypeOrPromise
loadSuperblock(): TypeOrPromise
- loadFile(inode: number): TypeOrPromise
+ readFile(inode: number): TypeOrPromise
writeFile(inode: number, data: Uint8Array): TypeOrPromise
wipe(): TypeOrPromise
close(): TypeOrPromise
}
```