Added @types/unist to package.json dependencies

[b-zurg]

This is a very minor change and adds the types provided by the @types/unist type declarations as a dependency for the basic unist types. I made this change because the current library was causing issues with tsc doing a lib type check and finding that the import in "unist-util-filter/types/index.d.ts" of Node
i.e.

import {node} from 'unist'

was breaking.

Specifically this is the error I'm resolving:

node_modules/unist-util-filter/types/index.d.ts:3:20 - error TS2307: Cannot find module 'unist'.

3 import {Node} from 'unist'
                     ~~~~~~~

node_modules/unist-util-is/index.d.ts:3:28 - error TS2307: Cannot find module 'unist'.

3 import {Node, Parent} from 'unist'
                             ~~~~~~~


Found 2 errors.

The only way to fix this was by adding in my project's tsconfig the setting "skipLibCheck": true which is not really an acceptable solution.

[wooorm]

You can also install @types/unist yourself as a fix if you are using types, right? That’s what we suggest for https://github.com/unifiedjs/unified

[ChristianMurphy]

We've avoided adding direct dependencies on @types/ packages due to the issues outlined in unifiedjs/unified#45 .
When using Typescript add the unist typings using npm install @types/unist.