Transformers take VFile instead of VFileCompatible

[clavin]

Currently, the types for this package define that Transformers accept a file parameter that is VFileCompatible. The spec, on the other hand, states the type of the file parameter is a VFile.

The current definition is unergonomic because a plugin defined as:

export default () => (node: Node, file: VFile) => {
  // ...
};

...cannot be passed to use since file: VFile is incompatible with file: VFileCompatible (ironically):

import plugin from '...';

unified().use(plugin);

causes:

Argument of type '() => (tree: Node, file: VFile) => Node' is not assignable to parameter of type 'Attacher<[], Settings>'.
  Type '(tree: Node, file: VFile) => Node' is not assignable to type 'void | Transformer'.
    Type '(tree: Node, file: VFile) => Node' is not assignable to type 'Transformer'.
      Types of parameters 'file' and 'file' are incompatible.
        Type 'VFileCompatible' is not assignable to type 'VFile'.
          Type 'string' is not assignable to type 'VFile'. ts(2345)

Even more, existing plugins built on the unified spec expect a VFile to be passed without needing to coerce the passed value into a VFile, making the current definition inaccurate for these plugins.

This PR fixes this issue by adding some basic tests that make sure this pattern works with use & making the necessary 10-character removal to make them pass.

[ChristianMurphy]

/cc @Rokt33r

[ChristianMurphy]

Thanks @clavin!
Looking at

unified/index.js

Line 295 in b07233d

transformers.run(node, vfile(file), done)

It appears Unified will always convert the vFileCompatible type to a vFile when handing off the the transforms.
This LGTM 👍

[Rokt33r]

It makes sense to me. Thanks for the contributing!!! ❤️

[Rokt33r]

I guess this can be deployed as a patch release because it is a kind of bug fix.