fix: inline math interpreted as attributes

[edomora97]

When using inline math ($e^{-x^2}$) the curly braces are interpreted as attributes by markdown-it-attrs. Since most of the times they are not valid attributes they simply get removed.

This patch escapes the curly braces (the default attribute delimiter), fixing the KaTeX rendering errors.

The problem occurs only when the inline math ends with a curly brace block (see examples below). For this reason the third example below works also in the current version.
An alternative fix may have been to force the math block not to end with the curly braces, for example by adding some spacing (e.g. \,). I preferred not adding some spurious spaces, the escaping strategy should be pretty invisible.

One could think it should be enough to escape the last { and }, in practice it's not enough: $e^{e^{x}+1}$ would become $e^{e^{{x}+1}}$, which is valid but clearly not equivalent. (note that this example is not broken in the current version since the last curly block contains another curly block, which apparently it's fine).

It would be nice to simply skip that rule for katex_inline block types but as far as I know markdown-it-attrs doesn't have such an option.

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Example

$e^{-x^2}$

$P_{static} = I_{DD}V_{DD}$

$V_{DD}=5V$

Previously

ParseError: KaTeX parse error: Expected group after '^' at position 2: e^̲
ParseError: KaTeX parse error: Expected group after '_' at position 21: …atic} = I_{DD}V_̲

Now

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Fixes #1581

[ivlydp]

It seems still have this problem in version 2.9.191
for rendering inline function such as $H_{ADAS2Eye}$
KaTeX parse error: Expected group after '_' at position 2: H_̲ ,
however in edit/preview mode it is right.
please help check it.