Replace String.prototype.substr() occurrences with String.prototype.substring()
As outlined in https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr, which refers to the ECMA-262 specification, using the `substr` function is advised against.
Hence this PR, which replaces all remaining `substr` occurrences with `substring` instead. Please refer to https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr#Syntax respectively https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substring#Syntax for the differences between the two functions.
Note that in most cases in the code-base there's only one argument passed to `substr`, and those require no other changes except replacing "substr" with "substring". For the other cases, the `substr(start, length)` calls are changed to `substring(start, start + length)` instead.
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