Be more strict in matching Drupal :var placeholders
Be more strict in matching Drupal :var placeholders

64aefe03 was too eager match Drupal's :var placeholders, resulting in
false positives on things like mailto:foo texts.

Per Drupal docs, :var is used mostly for URLs passed to href and should
always be quoted, i.e. in the form of href=":url" or href=':url'. Match
it accordingly and only highlight quoted uses of :var.

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