Support async getAllPaths() for remote filesystem backends

[manishrc]

Problem

IFileSystem.getAllPaths() is the only async-incompatible method in the interface (besides the pure resolvePath):

// Every other method returns Promise<...>
getAllPaths(): string[];  // ← sync only

This makes it impossible for remote filesystem implementations (Dropbox, S3, Google Drive, etc.) to support glob expansion (*.txt, /projects/*.md) without an upfront prefetch of the entire file tree.

Commands like find and grep -r work fine — they walk the tree via readdir() which is async. But glob patterns in word expansion call getAllPaths() synchronously, so remote backends must either:

1. Return [] (globs silently match nothing)
2. Require users to call a prefetch method before any glob — surprising and easy to forget

Proposal

Make getAllPaths() accept async implementations:

getAllPaths(): string[] | Promise<string[]>;

This is backwards-compatible — existing sync implementations (InMemoryFs, OverlayFs, ReadWriteFs) continue returning string[]. The glob expansion code in the interpreter just needs to await the result (it's already in an async context).

Context

This came up while building just-bash-dropbox, a Dropbox HTTP API backend for IFileSystem. Everything works except globs — find . -name "*.md" works (uses readdir), but cat *.md doesn't (uses getAllPaths).

Tracking issue: manishrc/just-bash-dropbox#1