fix(memfs): resolve relative streams from virtual cwd

[xianjianlf2]

Summary

• resolve Superblock relative paths against the volume's process-like cwd instead of the host process cwd
• give the default memfs export and memfs() helper a virtual-root cwd by default
• add regression coverage for relative createWriteStream paths on memfs() and the default fs export

Fixes #900.

Tests

• yarn build
• yarn jest packages/memfs/src/tests/memfs.process.test.ts --runInBand
• yarn jest packages/fs-node/src/tests/volume.process.test.ts --runInBand
• yarn prettier --check packages/fs-core/src/Superblock.ts packages/fs-node/src/tests/volume.process.test.ts packages/memfs/src/tests/memfs.process.test.ts packages/memfs/src/index.ts
• node runtime check for default fs.createWriteStream('relative') and memfs().fs.createWriteStream('relative')