Fix parsing of match pattern

[kit494way]

Fix #60

This PR changes the syntax tree of list destrcture pattern.

Before this change, only [$x ..$y] is supported as list destrcture pattern.
However, there are various cases (e.g. [1 ..$x], [$x $y ..]).
To support other various cases, this PR changes the way of parsing list destrcture pattern.

Before this change, parsing list destrcture pattern generates different syntax tree from list pattern.
For example, when parsing match $xs {[$x ..$y] => {}}, (val_list) and (val_variable) nodes were not generated under (match_pattern) node before this change as follows.

(nu_script
  (pipeline
    (pipe_element
      (ctrl_match
        (val_variable
          (identifier))
        (match_arm
          (match_pattern
            (identifier) ; <-- (val_list) and (val_variable) are not generated
            (identifier))
          (block))))))

After this change, the parsing is done without distinguishing between list destrcture patten and list pattern.
(val_list) and (val_variable) nodes are also generated when parsing list destrcture pattern as follows.

(nu_script
  (pipeline
    (pipe_element
      (ctrl_match
        (val_variable
          (identifier))
        (match_arm
          (match_pattern
            (val_list
              (val_variable
                (identifier))
              (val_variable
                (identifier))))
          (block))))))

[fdncred]

wow! nice work, thanks!