prefer-string-slice makes a breaking change for negative lengths

[sherlock1982]

Consider the following autofix:

"Sample".substr(0, "Sample".lastIndexOf('/')) returns empty string.
Due to ECMA-262 "If length is a negative number, it is treated as 0."

Unicorn changes it to:
"Sample".slice(0, "Sample".lastIndexOf('/')) returns "Sampl".

I'm not sure if autofix shouldn't be breaking.
But i think you need to be aware of this behavior.

UPD: I see that in other places it correctly added Math.Max for substring but substr seems broken.

[fisker]

/cc @voxpelli

[voxpelli]

Right, related change: #424

Not sure if I have time to look at this right now, but maybe