Dedicated JsonU128 type might bring more clarity to our standards

[frol]

NEPs currently use string for the amount fields, and that requires documentation to cover that it is actually an unsigned integer with u128 range.

Defining the JsonU128 type (a "string-encoded decimal number") and refactoring all the NEPs to use it might benefit the readability and consistency.

I don't have time to champion this change, but if someone is interested in this invaluable initiative, please, go ahead!