var separate syntax for variable

[cedric05]

Tests

• support $random & all such
• remove loading vars from comments (which is bad)
• add test for normal var
• add test for no default test for var
• add test for var set from env for var
• add test for var set from property for var
• add test for var set from import for var
• add test for override test
• add test for json/true/null/false/int/float var
• add import test for json/true/null/false/int/float var
• add notebook same cell test for json/true/null/false/int/float var
• add notebook different cell test for json/true/null/false/int/float var
• add notebook different cell import test for json/true/null/false/int/float var
• add notebook same cell import test for json/true/null/false/int/float var
• fix json handling with payload (either str or json)