Provide original_str in node bindings

[Pierrci]

When using the node bindings, there is no way for now to map back to the original string like it's possible in python or rust with output.original_str[output.offsets[1]]

[Pierrci]

@n1t0 I would go for a getOriginalString(begin?: number, end?: number) method with the same behavior than array.slice:

• If no parameter, return the entire original string
• If only first parameter, return the original string starting at the index
• If two parameters, return the original string between the two indexes (stopping before the second one)

wdyt?