prefer-string-slice: Improve autofix

[Promise2679]

closes #2505

I noticed this issue and attempted an implementation.

The rule now produces str.slice(0, -x) instead of str.slice(0, Math.max(0, str.length - x)) when x is a positive numeric literal.

Please let me know if there’s anything I should adjust.

[sindresorhus]

This still looks unsafe for non-interger numeric literals. The new shortcut accepts foo.length - 1.5 and fixes it to foo.slice(0, -1.5), but that changes behavior.

For example, with const value = 'abcdef', both value.substr(0, value.length - 1.5) and value.substring(0, value.length - 1.5) return abcd, while value.slice(0, -1.5) returns abcde. 0.5 is even worse because slice(0, -0.5) becomes an empty string.

This should only apply when the subtracted literal is a positive integer, not just any positive number.

[Promise2679]

Thanks for the review! I've fixed it by adding an isDecimalInteger check and included the corresponding test cases.