get/set on colums off by one

[lukaswagner]

The find predicate is wrong:
const chunk = this._chunks.find((c) => c.offset < index && c.offset + c.length >= index);
When reading index 0, no chunk is found, since the first chunk has offset 0, which is equal to the index. Thus, the lower check has to be changed to <=. Accordingly, the upper check has to be changed to >, as offset + length is already outside the chunk.