Preserve comment placement when trailing a name clause

[lionel-]

Progress towards solving the idempotence panic mentioned in #434
Progress towards #435

Comments in:

foo(
  name = # comment
    value
)

foo(
  name =
    # comment
    value
)

are currently placed on the RArgument node. This causes them to move higher in the tree, and they get formatted as:

foo(
  # comment
  name = value
)

foo(
  # comment
  name = value
)

This displacement is particularly problematic in:

list(
  foo  # comment1
  =1, bar  # comment2
  =  # comment3
  2,
)

as the comment ordering becomes:

list(
  # comment1
  foo = 1,
  # comment3
  # comment2
  bar = 2,
)

To solve this, the PR:

• Places trailing name clause comments as leading comments of argument values
• Introduces a line break and an indent in arguments with both name clauses and leading comments on values

This solves the ordering issue in the last example which is now formatted as:

list(
  # comment1
  foo = 1,
  # comment2
  bar =
    # comment3
    2,
)

Remaining issues: in the following example, comment 6 is still formatted out of order, and comment 7 is still on the same line as comment 5:

# fmt: table
list(
  # comment0
  foo  # comment1
  =1, bar  # comment2
  =  # comment3
  # comment4
  2 #comment5
  #comment6
  , #comment7
  #comment8
)

becomes:

# fmt: table
list(
  # comment0
  # comment1
  foo = 1,
  # comment2
  bar =
    # comment3
    # comment4
    2, #comment5 #comment7
  #comment8
  #comment6
)