Sublime: Add option to specify env vars for the .sublime-project

Create a `CMAKE_SUBLIME_TEXT_2_ENV_SETTINGS` variable to control
addition of env vars in the `.sublime-project`.

Closes: #16387

Source/cmExtraSublimeTextGenerator.h

@@ -66,6 +66,7 @@ private:
                              cmGeneratorTarget* gtgt);
 
   bool ExcludeBuildFolder;
+  std::string EnvSettings;
 };
 
 #endif