Add index.d.ts

[guillaumegarcia13]

Subject of the feature

As for the other unist-util-*, please consider adding index.d.ts

[ChristianMurphy]

We are in the process of adding types, you can see some examples of types being added in
syntax-tree/xast-util-sitemap#1, syntax-tree/hast-util-sanitize#21, and syntax-tree/mdast-util-to-string#5
Note that for types as a dts file, using DTSLint and/or type coverage is highly recommended.

More recently there has been a push to get types generated from JSDoc
wooorm/zwitch@88b233d and wooorm/zwitch#6

Either works, and a PR would be welcome!

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Closing this issue as we don't want to keep a types tracker issue in each of the 300+ unified repos.