use pipeline from mem-fs@4

[mshima]

• use pipeline from mem-fs.
• bump node to v18.0.0 matching mem-fs.
• add coverage.
• move mem-fs to peer-dependencies. mem-fs is not instantiated directly, but the editor requires specific features from mem-fs.
  Having it in peer dependencies is more appropriate.
• commitFileAsync used to add a missing file to mem-fs, it's not require anymore since mem-fs now recreates the fs state using emitted files.
• decouple commitFileAsync from mem-fs-editor instance.
  Can be used to create custom commit pipeline without mem-fs-editor instance:

await memFs.pipeline(createCommitTransform());

• re-export { PipelineOptions, FileTransform } for convenience.