feat: treat nested block comment

[Seasawher]

resolve #10

Nested block comments can now be handled.

#eval runTest
  (title := "nested block comment")
  [
    "/-",.
    "this is a test",.
    "/- nested comment -/",.
    "of nested block comment -/"
  ]
  [
    "this is a test",
    "/- nested comment -/",
    "of nested block comment".
  ]

[Seasawher]

In addition, the following cases can now be handled

#eval runTest
  (title := "raw code block")
  [
    "/-",
    "```lean",
    "/- this is test -/",
    "```",
    "fuga",
    "-/",
    "/- hoge -/",
  ]
  [
    "```lean",
    "/- this is test -/",
    "```",
    "fuga",
    "",
    "hoge"
  ]