format: retain comments in expression bodies (value decls, let, lambda)

[avh4]

After this PR, https://github.com/gren-lang/example-projects can now be safely formatted! (At least the main branch; I'll check 0.2 branch next)

• retain comments within top-level value declarations...
  • before each argument pattern
  • before =
  • between = and the body
  • after the body (indented, before the next freshline)
• retain comments within let expressions...
  • between let keyword and first declaration
  • between declarations
  • after declaration bodies (indented)
  • between last declaration and in keyword
  • between in keyword and body
• retain comments within lambda expressions...
  • between \ and first arg
  • between args
  • between last arg and ->
  • between -> and body

[robinheghan]

Would you mind merging with main before I review?

[avh4]

Ready!