Modularization, toSpec, test and more

[srizvi6]

-Modularized code so that it is easier to read + generatorVL.ts is easier to read/use.

• Moved ASTStatement(to be renamed) generation to parser.ts.
• Created generate.ts that decouples VL specific generation from class/method generation.
• Added Spec class with toSpec() function.
• Added a test (tests/test_mark_spec.ts) that checks if the json is being created. Currently able to get {"mark":{"type":"bar"}} from the generated VL code.

Next steps:
-Clean up api.ts for a more programmatic approach to VL specific logic.
-Need to change the way method args are handled and create an object/tuple for arg (keyword, name, type).
-Add Encode, Position, Color classes.

[srizvi6]

Closing to clean up issues list