Refactor rendering submodules to sort by syntax instead of by parser/renderer

Currently we have : xwiki-rendering/xwiki-rendering-parsers/xwiki-rendering-parser-wikimodel

It would be nicer to have xwiki-rendering/xwiki-rendering-syntaxes/ followed with project containing both parsers and renderers like:

xwiki-rendering/xwiki-rendering-syntaxes/xwiki-rendering-syntax-wikimodel
xwiki-rendering/xwiki-rendering-syntaxes/xwiki-rendering-syntax-xhtml
xwiki-rendering/xwiki-rendering-syntaxes/xwiki-rendering-syntax-xdom
...

this because parsers and renderer usually share informations and are synchronized.