Fix `String.last` & `String.first` errors

[omaus]

• Fixes #21
• Adds test cases regarding #21

[kMutagene]

I'd suggest raising the same error messages as built-in functions, which use invalidArg: https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/exception-handling/the-invalidarg-function

e.g. invalidArg (nameof str) "The input string was empty"

Examples: