Draft: Allow trailing triple-slash comments in Prisma

[cprecioso]

This is a currently-not-working PR for allowing trailing documentation comments in a node when parsing PSL:

model Post {
  title String /// <- this trailing comment should get attached to the `title` node
}

Right now (as of #2949) we only support leading comments.

---

Problem

#2949 (comment)

Solution

I asked at Haskell's Matrix Room, and got the suggestion that PSL is a newline-significant language, so it is common to split the lexeme parser into one that consumes whitespace and another that consumes whitespace+newlines.

In the end I made two spaceConsumer parsers, and use the non-newline version everywhere, and the yes-newline version whenever it makes sense (in between top-level structures in the Prisma file, and inside lines in { braces }).

I kept debugging and tweaking but at some point I had to stop working on it as it is way eating into the time budget for this task.