Recognize "oksh" executable name as ksh

A portable version of OpenBSD's ksh is distributed with the executable
name oksh [1]. It's a descendant of pdksh and can be shellchecked as
ksh.

[1]: https://github.com/ibara/oksh

src/ShellCheck/Parser.hs

@@ -3387,7 +3387,8 @@ readScriptFile sourced = do
         "busybox sh",
         "bash",
         "bats",
-        "ksh"
+        "ksh",
+        "oksh"
         ]
     badShells = [
         "awk",