…t compatibility

The current implementation only accepts encoding as a positional parameter,
but Node.js fs.readFile/readFileSync accept either a string encoding OR an
options object with { encoding: 'utf8' }.

Libraries like isomorphic-git call fs.readFile with an options object:
  fs.readFile(path, { encoding: 'utf8' })

This was causing the encoding to be passed incorrectly to memfs, resulting
in Buffers being returned even when 'utf8' encoding was requested, leading
to 'X is not a function' errors when string methods were called on Buffers.

Changes:
- Updated readFile signature to accept BufferEncoding | { encoding?: BufferEncoding }
- Updated readFileSync signature to accept BufferEncoding | { encoding?: BufferEncoding }
- Extract encoding from options object if needed before passing to memfs
- Ensures compatibility with both calling conventions

This fixes issues when using isomorphic-git with memfs in browser environments.

…-option

fix: handle options object in readFile/readFileSync for isomorphic-git compatibility W-19263112