fix(parser): ignore or push out trivia tokens around arguments and groups

[Myriad-Dreamin]

Fix wrong token eat function. It introduces an additional change:

• Spaces around arguments, inside of the command, are ignored. For example,

  \frac { 1 } { 2 }

  is parsed as

  before,

  root
  |cmd
  ||cmd-name("\\frac")
  ||space'(" ") // ignored
  ||args { ... }
  ||space'(" ") // ignored
  ||args { ... }
  

  after,

  root
  |cmd
  ||cmd-name("\\frac")
  ||args
  |||curly
  ||||lbrace'("{")
  ||||space'(" ")
  ||||text(word'("1"),space'(" "))
  ||||rbrace'("}")
  ||args
  |||curly
  ||||lbrace'("{")
  ||||space'(" ")
  ||||text(word'("2"),space'(" "))
  ||||rbrace'("}")