Token methods lack checks for negative token amounts

[shuse2]

Description

Partially resolved in PR #8559. it resolved most instances of the problem, but the transferCrossChain function does validate its messageFee argument and the payMessageFee function does not validate its fee argument. Ideally, it would be great to use a new type that only supports unsigned bigints, but JavaScript does not support unsigned bigints by default.

Which version(s) does this affect? (Environment, OS, etc...)

6.0.0-beta.7