Invalid string offset usage

Reports invalid string offset usages that will result in a runtime error.

The following usages are reported:

• Using string offsets as objects or arrays

• Unsetting string offsets via unset

• Passing string offsets by reference

• Creating references to or from string offsets

• Incrementing or decrementing string offsets

• Using combined operator assignments such as += or .= on string offsets

• Assigning string offsets with empty strings or strings with length >1