Split out CaseSensitive enum out of catch_common.hpp

Only very few places actually use it, so there is no need to have
it be central to everywhere.

src/catch2/internal/catch_case_sensitive.hpp

@@ -0,0 +1,12 @@
+#ifndef CATCH_CASE_SENSITIVE_HPP_INCLUDED
+#define CATCH_CASE_SENSITIVE_HPP_INCLUDED
+
+namespace Catch {
+
+    struct CaseSensitive {
+        enum Choice { Yes, No };
+    };
+
+} // namespace Catch
+
+#endif // CATCH_CASE_SENSITIVE_HPP_INCLUDED