Fix regex patterns for inline double dollar

[rambip]

Fixes #2805

Here is what happened before with the input hello $$x$$:

1. The RE_DOLLAR_INLINE regex matches, but only on the internal expression ($x$). It is converted to html tags, but with the external dollars that still exist.
2. There is a second match.. The internal content is treated as is, but tags are appended a second time.

The fix was as easy as adding an optional additional dollar in the expression.

Now, all of these should match as before:

$x$
$$x$$
$x + y$
$$x + y$$
$a^2 + b^2$
$$\frac{1}{2}$$
text $x$ text
text $$x$$ text
$x$y$z$

But for $$x$$ and text $$x$$ text, the capture group should include the dollars.

[facelessuser]

Closing as a duplicate of #2807