Add Code Block to represent code content

A lot of syntaxes have ways to represent code blocks (in html it's the code tag, in markdown it's by indenting, etc).

We should consider having a CodeBlock in XWiki Rendering and possibly a syntax too in the XWiki Syntax.

This will make it better to support input syntaxes such as HTML or Markdown (right now we generate a Code Macro Block when we find a code block in Markdown, which isn't nice since the Code Macro is optional).