Start scripts using "script" rather than "def script"

[AndreVanDelft]

For now a script or scripts section starts with "def script" whereas "script" would be much nicer. The reason is that "script" is still an identifier, rather than a keyword, and it would be quite hard (if I am right) to see in Scala expressions etc that this special identifiers starts a new script definition.
A solution would be to ensure that "script" is scanned as a keyword. However this should be done conditionally, i.e. after "import subscript.language" or something like that.