Proposal: tagType.parser

cstml.cstml

<!docype cstml>
<!parsers>
  <CSTML "https://url/to/grammar"/>
  <Regex "https://url/to/re/grammar"/>
</>
<CSTML:TokenTag [Tag]>
  <| Punctuator "<|" balanced="|>" startSpan="Tag" |>
  <Identifier path="tagName">
    <| Identifier "ID" |>
  </>
  <| Trivia " " |>
  <RegexMatcher [ValueMatcher] path="value">
    <Regex:Regex [Regex] path="value">
      <| Punctuator "/" balanced="/" startSpan="Expression" |>
      <Pattern [Pattern] path="pattern">
        <Alternative [Alternative] path="alternatives">
          <Character [Element] path="elements">
            <| Literal "a" |>
          </>
        </>
      </>
      <| Punctuator "/" endSpan="Expression" |>
    </Regex:>
  </>
  <| Punctuator "|>" endSpan="Tag" |>
</CSTML:>

js.cstml

<!docype cstml>
<!parsers>
  <JS "https://url/to/grammar"/>
  <Regex "https://url/to/re/grammar"/>
</>
<JS:ExpressionStatement [Statement]>
  <RegExpLiteral [Expression] path="expression">
    <| Punctuator "/" balanced="/" startSpan="Expression" |>
    <Regex:Pattern [Pattern] path="pattern">
      <Alternative [Alternative] path="alternatives">
        <Character [Element] path="elements">
          <| Literal "a" |>
        </>
      </>
    </Regex:>
    <| Punctuator "/" endSpan="Expression" |>
  </>
</JS:>