Idempotence bug with indented multi-line haddocks

[brandonchinn178]

Is your bug specific to Fourmolu?

• Yes, I've verified that Ormolu does not have this bug

Describe the bug
#255 made it so that a leading space in single line comments don't count as part of a comment, but it causes "AST of input and AST of formatted code differ" on indented haddocks:

{- | Test

   3 spaces before this line
-}
foo :: Int

I assume this is because {- -} haddocks are all included as one long "Test\n\n 3 spaces before this line" string and we don't ignore leading whitespace after newline characters

For now, a workaround is to put the first line on the next line. No idea why this works.

{- |
Test

   3 spaces before this line
-}
foo :: Int