Allow tailing comma in arguments

[zhuliquan]

allow the last argument append with comma in func expr.
make below expr can be parsed ok.

func1(
     argument1,
     argument2,
     argument3,
)

[antonmedv]

Althoug this is nice feature, I think for not is not needed in Expr. as most of the time expressions is one line.

We may return later to this and change our decision.

[zhuliquan]

Althoug this is nice feature, I think for not is not needed in Expr. as most of the time expressions is one line.

We may return later to this and change our decision.

Although most of the time the expression is a single line, I still think this feature makes sense. This is considered from the perspective of syntactic consistency of expr. Because I discovered that expr parser supports ignoring the last comma on type of array or type of map, which is much like Python unlike JSON. For example, the following statements can be parsed via expr parser

[
   1,
   2,
]
{
  "1": 1,
  "2": 2,
}

So from this point of view, many users may naturally think that the last comma will be ignored by the parser. However, if the user finds that the function call is not allowed to add a comma at the end. This can be confusing.

[antonmedv]

Hmm. This is true. We do allow comma in arrays & maps. Let me think on it bit more.

[antonmedv]

Well, let's add support for this!