feat(pick): accept a callback for advanced picking

[aleclarson]

Tip

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Description

const newObject = pick(object, (value, key, object) => key.startsWith('a'))

This brings lodash.pickBy functionality to Radashi.

Checklist

• PR feat: add reduceKeys function #29 is merged
• Changes are covered by tests if behavior has been changed or added
• Tests have 100% coverage
• If code changes were made, the documentation (in the /docs directory) has been updated

Resolves

sodiray/radash#157
sodiray/radash#402

Also fixes sodiray/radash#326, because the array of keys can now be typed readonly without complaints from TypeScript.