use flatMap() from lodash instead of native

[vladislavkovaliov]

Issue: Not having polyfill of flatMap() it'd be good using it from lodash.

[joealden]

flatMap looks to be supported by all browsers that this library explicitly supports, so I disagree that this is needed.

• https://caniuse.com/mdn-javascript_builtins_array_flatmap
• https://github.com/Eliav2/react-xarrows/blob/197326ccb032d5b5a3a7e18def596abac7327b05/package.json#L71-L76