Feature: Documentation support to Enum AST macro nodes

[awr1]

Potentially related to #12353.

proc thing() =
  ## This doc comment will get dumped
  discard
type Foo = enum
  ## Top-level description for Foo
  bar ## Field comment
  baz ## Another field comment
  qux

Appropriate documentation comments will get generated for both the procedure and the enum, however when we use dumpTree:

StmtList
  ProcDef
    Ident "thing"
    Empty
    Empty
    FormalParams
      Empty
    Empty
    Empty
    StmtList
      CommentStmt "This doc comment will get dumped"
      DiscardStmt
        Empty
  TypeSection
    TypeDef
      Ident "Foo"
      Empty
      EnumTy
        Empty
        Ident "bar"
        Ident "baz"
        Ident "qux"

There appears to be no available way for a macro (if there is, it is undocumented and doesn't work with dumpTree) to be able to generate doc comments for enums, either in terms of the top-level description of the enum or comments on each of the enumerator values.

Adding doc comment support to enum nodes will give users fuller controller over the generated documentation that comes out of code produced by a marco.

[blackmius]

I found that comments can be accessed from global comment store

import compiler/[ast, parser, idents, options]
import deques

var stream = """
## dsfsdfdsf
type
  ## dsgsdgsdg
  B = enum
    ## enum doc
    B1 ## first kind
    B2 ## second kind
    B3 = 5 ## third kind
  A = object
    ## object doc
    ## sdfsdf
    ## dfsdfdf
    ## dsfsdfds
    field1: int ## documentation comment
    field2: string ## multi
                ## line
                ## doc
                ## string
    when defined(something):
    field3: int ## when doc
    case kind: B
    of B1: field4: int ## bla-bla
    of B2: field5: string ## ble-ble
"""

var cache = newIdentCache()
var config = newConfigRef()

let a = parseString(stream, cache, config, "test.nim", 0)

const untraversable = { nkCharLit..nkUInt64Lit, nkFloatLit..nkFloat128Lit, nkStrLit..nkTripleStrLit, nkSym, nkIdent }

proc findComments(node: PNode) =
  var queue = initDeque[PNode]()
  queue.addLast(node)
  while queue.len != 0:
    let node = queue.popFirst()
    if node.comment != "":
      echo node.kind, ' ', node.comment
    if node.kind in untraversable:
      continue
    for i in 0..<node.len:
      queue.addLast(node[i])

findComments(a)

they are assigned every time the parser thinks there will be a comment
https://github.com/nim-lang/Nim/blob/devel/compiler/parser.nim#L203
and stored in https://github.com/nim-lang/Nim/blob/devel/compiler/ast.nim#L1025
which can accessed later