strip: properly remove math in all scenarios

[crowlKats]

deno-gfm/example/content.md

Lines 68 to 69 in 79ade25

	$$ \begin{aligned} \dot{x} & = \sigma(y-x) \\ \dot{y} & = \rho x - y - xz \\
	\dot{z} & = -\beta z + xy \end{aligned} $$

is not properly handled

[sanderhahn]

Also noticed that the regex for the block formulas don't allow newlines as whitespace. The VSCode Markdown Preview Enhanced extension also allow more whitespace in inline formulas.

$$
x
:=
x
$$

$$ x := x $$