Investigation: `bn.js` vs `bignumber.js` vs `bigInt`

Currently we have many versions of `bn.js` and a few of `bignumber.js`. We need to investigate the following:

1. Can these packages be used interchangeably?
2. If so, which one should we use?
3. Can we eventually migrate use to `bigInt`? Or do we rely on functionality not available with `bigInt`

Search the extension for imports and usages of bn.js, bignumber.js, and BigInt, then compare the operations those call sites require. Done means documenting whether the packages are interchangeable, recommending one library, and identifying any functionality that prevents migration to BigInt.

Written by the indexing model from the issue text.