DollarMathCloser class

Finds the closing $ of single-dollar maths, for one text.

Pandoc's rule, which is what keeps prices out of maths: the opening $ must have a non-space right after it, and the closing one a non-space right before it and no digit right after. $x$ and $\frac{a}{b}$ are maths; $5 and $10 has no closer, so both dollars stay prose. An escaped \$ and either half of a $$ never close. With sameLine the closer must be on the opener's line.

One addition: an opener followed by a digit closes only before the next whitespace. $2^{10}$ is maths, but cost $5 and $x^2$ is the price $5 followed by the formula x^2 — Pandoc alone reads 5 and $x^2 as one formula. The streaming reveal never opens at a digit, so the two agree on every price.

The one rule the parser, the $ rewrite and the streaming reveal share. Openers must be asked about left to right: a search that fails is remembered, because every later opener (on the same line, with sameLine) would fail too. Without that a line of $a repeated costs a full scan per dollar.