Support mode to customize Main module

[brandonchinn178]

Support an advanced mode that only generates the imports + test tree, e.g.

{- AUTOCOLLECT.MAIN

# defaults to false
custom_main = true

# these are still relevant
group_type = ...
strip_suffix = ...
-}

{- AUTOCOLLECT.MAIN.imports -}
import Test.Tasty

main :: IO ()
main = do
  preProcessingSteps
  defaultMainWithIngredients customIngredients tests

tests :: TestTree
tests = id {- AUTOCOLLECT.MAIN.tests -}

This will support NoImplicitPrelude or custom preludes