add option to enable inline math $…$

[SichangHe]

add option to enable inline math $…$

We only support \\(…\\) for inline math right now. This PR adds an option to enable using $…$ for inline math.

usage

add this following to book.toml to enable this feature:

[output.html]
mathjax-support = true
inline-math-dollar-sign = true

unwanted usage

If the user enables this option without enabling MathJax support, nothing happens:

[output.html]
inline-math-dollar-sign = true

benefits

This will be helpful for writers that want to use a lot of inline math in their books, especially if it is a Mathematics book.

In equation [1], $x$ is …, $y$ is …

Looks much more friendly than:

In equation [1], \\(x\\) is …, \\(y\\) is …

It is also much easier to type.

Some extensions (for example, Markdown All in One for VSCode) gives a keyboard shortcut to add two $$ and put your cursor in the middle. This is very convenient.

drawback

If this option is enabled, the user will need to escape all their regular dollar signs:

\$

Therefore, this option is set to false by default.

[SichangHe]

This makes no sense to me now because anyone that writes a lot of math with mdbook should probably use the mdbook-katex preprocesser unless mdbook has proper \ support in math. The MathJax support in mdbook is extremely clunky because all the \ needs to be written as \\, which makes it incompatible with any other markdown math renderer and makes the code look bad.

The takeaway is probably that we need a "math" section in the mdbook book that tells the user that they should opt in mdbook-katex for a better math support, or we should include an easy way to use their features.

[expikr]

Additionally, for those who want to use "vanilla" mdbook, if #2069 is accepted then the inline delimiter becomes the very concise \`...\` using the even more concise AsciiMath syntax.