Implement method `CppStringT::contains()`

[schmouk]

This is an equivalent of the Python operator in.
Notice: This method is available in c++23 :

constexpr bool
contains( std::basic_string_view<CharT,Traits> sv ) const noexcept; (1) 	(since[ C++](https://en.cppreference.com/w/cpp)23)

constexpr bool
contains( CharT ch ) const noexcept; (2) 	(since C++23)

constexpr bool
contains( const CharT* s ) const; (3) 	(since C++23)

[schmouk]

Completed. Validated.