Use 1 split iterator to split while ignoring `\`

[SichangHe]

Use iterator SplitIgnoreEscaped to split these ignoring the ones prefixed with \:

• ``` to handle code blocks.
• ` to handle inline code blocks.
• $ and $$ to render math expression.

The rule is simplified to "escape whatever that is prefixed with \."

Notice

This changes the behavior on handling \$ and \$$. Previously, \$ was replaced as $, \$$ was not escaped. Now, both of the above would be preserved as is.

[SichangHe]

This change would mean that if someone has:

$$f(x)\\$$
and …

then, it would break because the \ is right before $$.

But, there are no obvious reason to do the above because creating a trailing new line inside math expressions is not really useful.

[SichangHe]

This change also makes the preprocessor slightly faster.