Adds StringSyntaxHighlighter

[simonbs]

This PR adds StringSyntaxHighlighter which can be used to syntax highlight a string without needing to create a TextView.

let syntaxHighlighter = StringSyntaxHighlighter(
    theme: TomorrowTheme(),
    language: .javaScript
)
let attributedString = syntaxHighlighter.syntaxHighlight(
  """
  function fibonacci(num) {
    if (num <= 1) {
      return 1
    }
    return fibonacci(num - 1) + fibonacci(num - 2)
  }
  """
)

The StringSyntaxHighlighter is showcased in the example project living in the simonbs/RunestoneSnippetExample repository.

The PR is marked as a draft while I verify that the StringSyntaxHighlighter addresses a need.