cmStringAlgorithms: Move string functions to the new cmStringAlgorithms.h

This adds the `cmStringAlgorithms.h` header and moves all string functions
from `cmAlgorithms.h` to `cmStringAlgorithms.h`.

Source/cmUnsetCommand.cxx

@@ -2,8 +2,8 @@
    file Copyright.txt or https://cmake.org/licensing for details.  */
 #include "cmUnsetCommand.h"
 
-#include "cmAlgorithms.h"
 #include "cmMakefile.h"
+#include "cmStringAlgorithms.h"
 #include "cmSystemTools.h"
 
 class cmExecutionStatus;