fix: enforce 0x prefix for numeric hex strings

[ruvaag]

Motivation

Resolve #5808 to enable disambiguous numeric hex string parsing

Solution

• Explicitly check that numeric strings being parsed via *parseInt or *parseUint cheats don't contain any non-numeric characters
• Throw if the above does not hold
• Modify test added in test: add test for #5808 #5809 to verify new behavior

[mattsse]

this looks correct, tests look good

tysm