Test Parameters in Various Contexts

[tajmone]

Ensure parameters are correctly detected and handled in all their valid usage
contexts (See: pml-lang/pml-userman#2):

• the value of an attribute (e.g. title="foo<<p1>>bar")
• text contained in a node, including nodes that contain raw text, such as code or html. (e.g. [div foo<<p1>>bar])
• the value of a constant — i.e. a parameter declaration can use the value of a previously defined parameter in its value (e.g. [const p2 = foo<<p1>>bar])

I'll need to add specific syntax texts to ensure parameters are honoured.