Split out CaseInsensitiveCmp into its own file

src/catch2/internal/catch_case_insensitive_comparisons.cpp

@@ -0,0 +1,26 @@
+
+//              Copyright Catch2 Authors
+// Distributed under the Boost Software License, Version 1.0.
+//   (See accompanying file LICENSE_1_0.txt or copy at
+//        https://www.boost.org/LICENSE_1_0.txt)
+
+// SPDX-License-Identifier: BSL-1.0
+
+#include <catch2/internal/catch_case_insensitive_comparisons.hpp>
+#include <catch2/internal/catch_string_manip.hpp>
+
+#include <algorithm>
+
+namespace Catch {
+    namespace Detail {
+
+        bool CaseInsensitiveLess::operator()( std::string const& lhs,
+                                              std::string const& rhs ) const {
+            return std::lexicographical_compare(
+                lhs.begin(), lhs.end(),
+                rhs.begin(), rhs.end(),
+                []( char l, char r ) { return toLower( l ) < toLower( r ); } );
+        }
+
+    } // namespace Detail
+} // namespace Catch