[utils] `isHexChecksumAddress` is poorly named and documented

https://github.com/MetaMask/utils/blob/1bea7f378c37117260912af1921556bc57110d20/src/hex.ts#L63-L71

This function does NOT return if the given string is a "valid hex checksum address" (or a "Hex Checksum Address"). It returns true if the _characters_ used in the given string are all allowed in an checksummed address.

The function (and likely some other similarly confusing functions/docs in this same file) should be updated to reflect their purpose.

Start with src/hex.ts lines 63-71 and inspect the similarly confusing functions and documentation in that file. Update the naming and documentation to describe the character validation behavior rather than checksum-address validity, with all affected descriptions accurately reflecting their purpose.

Written by the indexing model from the issue text.