Behavior for negative index values is not specified for 4 functions

[ast-hugger]

The following functions:

• fromCharCodeArray
• intoCharCodeArray
• charCodeAt
• codePointAt

feature parameters of type i32 interpreted as string indices (start, end, or index, depending on the function). The existing specification does not state what should happen if a negative value is passed as one of these parameters.

[sjrd]

The "JS specs" in the Overview of the proposal are clear: they interpret all those arguments as unsigned integers, and therefore no negatives can occur.

The "Modified spec", however, does not have corresponding steps (like "Let start be ToUint32(start)"). So I think we need to fix the modified spec to adjust to the overview.