Allow `wkg fetch` to specify manifest directory

What do you think abou workspace root dir as a property w/ a default to current dir when it's missing?

_Originally posted by @vados-cosmonic in https://github.com/bytecodealliance/wasm-pkg-tools/pull/231#discussion_r3572587216_

Start at the wkg fetch command entry point and trace how it locates the manifest; the issue names no files or tests, so first clarify whether the workspace-root property is a fetch option and how it relates to the current directory. Done means wkg fetch can use the specified manifest directory and still defaults to the current directory when no value is provided.

Written by the indexing model from the issue text.