Updated existing and added new commands to library and added a new TextNode class which is representing raw Text
CodeSharper.Core/Texts/TextNode:
  - Added and tested TextNode class
  - Represents raw text in source code
CodeSharper.Core/Common/Commands/StringTransformationCommand:
  - Added a new abstract class which is common base class of string transformation commands
CodeSharper.Core/Common/Commands/ToLowerCaseCommand and ToUpperCaseCommand:
  - Added two string transformation command