Document `witPath` option

[rylev]

The witPath option to the componentize function can either be a path to a single wit file or to a directory. If the path is a wit file then the wit package must not have any dependencies. Otherwise, if the path is a directory, the package's dependencies can be located in a deps directory.

This should be documented as it is not obvious that this is the behavior.

[guybedford]

With the type definitions now landed, there is a basic description in https://github.com/bytecodealliance/ComponentizeJS/blob/main/types.d.ts#L17. If you'd like to improve that contextual documentation with a better description, I'll gladly merge any PR.