fix: correctly highlight text verbatim

[mochja]

Back-porting update from vs-code:

• fix: correctly highlight text verbatim grafana/vscode-jsonnet#28

Eventually github will correctly highlight these examples:

{
  a: |||
 hello
|||,
  b: 0
}

{
 a: |||
  hello
 |||
}

{
 a:
  |||
  this is not terminated text block, but renders as if it is
  |||,
 b: true
}

{
 a:
  |||
  this is valid example but does not work
 |||,
 b: true
}

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