fix: use HTML Living Standard comment syntax

[acid-chicken]

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Currently, double-hyphens (--) inside comments aren't allowed in the linguist syntax highlight. However, the parser allows them without any warnings.

The SFC Spec says "use HTML comment syntax" but HTML comment syntax definition is little ambiguous. HTML Living Standard allows to use double-hyphens inside, however, contrarily HTML 4.01 and past forbid it. Current vue-syntax-highlight seems refering the latter spec, but the actual parser implementation follows the former spec.

This change adjusts vue-syntax-highlight definitions to vue's compiler/parser/html-parser behavior.

[acid-chicken]

found a related PR on Vetur, fyi: vuejs/vetur#1052