test: add property tests

[ChristianMurphy]

this introduces fast-check (https://dubzzz.github.io/fast-check.github.com/) as a way to automatically generate test cases exploring possible edge cases not covered in example based tests

[wooorm]

This seems like a very different test type. If so, I’d prefer having the current file as test/main.js, the additions as test/property-based.js, and a test/index.js tying them together.

I think bigint shouldn’t be supported in unist, because it can’t be roundtripped with JSON (https://github.com/syntax-tree/unist#node)

The last check looks complex: first a long line (.chain) where it makes a certain format, then a long line (is) where it undoes that work?

[ChristianMurphy]

first a long line (.chain) where it makes a certain format

An arbitrary JSON object. 7ae6376 and 62b6329 try to clarify this

then a long line (is) where it undoes that work?

7ae6376 clarifies this some by splitting up the massive line.
It specifically generates an object with a subset of properties from the original node

[wooorm]

good to go once CI succeeds!