Fix #315 - RemoveIndexSignature deprecated in v4

`RemoveIndexSignature` added in `type-fest` [v2.10.0](https://github.com/sindresorhus/type-fest/releases/tag/v2.10.0). Renamed to `OmitIndexSignature` in [v4.0.0](https://github.com/sindresorhus/type-fest/releases/tag/v4.0.0). `package.json` updated to reflect compatibility.