CodeFormatter: Implement proper Comment handling.

[ldoblies]

Comments are not fully supported yet: They are only properly formatted when they don't interfere with postNewLine()s. It might be necessary to pre-process the AST and move
the WS nodes containing comments to the correct node. Then it is possible to e.g. post a newline after clauses in a FLWORExpr.

Example:

let $x := 1 (: comment within a flwor :)
(: another one :) return $x

Formats to:

let $x := 1
(: comment within a flwor :) (: another one :) return $x

Both comments are in the same WS-node between the LetClause and the ReturnClause.
It would be necessary to make the first Comment a child of the LetClause and the second one a child of the ReturnClause. Then the formatting rules for FLWORExpr/Statement (and all others) could be kept as they are now.