Adds attribute_group node to complete attributes support

[cfroystad]

Also fixes a bug with missing support for trailing comma in attribute listings

This changes the node structure for currently unreleased code.

Example

Code:

#[
    A1,
    A2(),
    A3(0),
    A4(x: 1),
]
function a() {
}

Before:

(function_definition
    attributes: (attribute_list
        (attribute (name))
        (attribute
          (name)
          parameters: (arguments)
        )
        (attribute
          (name)
          parameters: (arguments (argument (integer)))
        )
        (attribute
          (name)
          parameters: (arguments (argument name: (name) (integer)))
        )
    )
    name: (name)
    parameters: (formal_parameters)
    body: (compound_statement)
  )

After:

(function_definition
    attributes: (attribute_list
      (attribute_group
        (attribute (name))
        (attribute
          (name)
          parameters: (arguments)
        )
        (attribute
          (name)
          parameters: (arguments (argument (integer)))
        )
        (attribute
          (name)
          parameters: (arguments (argument name: (name) (integer)))
        )
      )
    )
    name: (name)
    parameters: (formal_parameters)
    body: (compound_statement)
  )

Checklist:

• All tests pass in CI
• There are sufficient tests for the new fix/feature
• Grammar rules have not been renamed unless absolutely necessary (0 rules renamed)
• The conflicts section hasn't grown too much (0 new conflicts)
• The parser size hasn't grown too much (master: 2330, PR: 2338)