Allow blockly to use environment variables in Velocity

When executing Velocity, multiple environment variables are set depending on the context: $doc, $context, $xwiki, $services, …

It is critical to have access to those variables when executing Velocity as it will allow us incredibly cool tricks : loading a document, sending a mail, … basically interacting with all the components of XWiki.

The best way to add that feature would be probably to create a new block for environment variables, acting as standard variables, but not set to $NULL at the beginning of the script. If it is not compatible with the Blockly data model, then we'll have to find a workaround.