Flatten an object to dotted paths

Form libraries key errors by "address.postcode". Translation files ship nested but get looked up by dotted key. Both need the nested-to-flat conversion.

Implement flattenObject(source). Walk the object recursively and return a single-level object whose keys are the dotted paths to each leaf and whose values are those leaves. Only plain objects are recursed into. Arrays are leaves and are stored as they are, not indexed into. null is a leaf, even though typeof null is "object". A nested object with no keys of its own is also kept as a leaf under its own path, so { a: {} } flattens to { "a": {} }. Flattening an empty object gives an empty object. Never modify source.

flattenObject({ a: { b: { c: 1 } }, d: 2 });
// { "a.b.c": 1, d: 2 }

flattenObject({ tags: [1, 2], meta: { ok: true } });
// { tags: [1, 2], "meta.ok": true } — arrays are leaves, not indexed

flattenObject({ a: {} });
// { a: {} } — an empty nested object survives as its own leaf