Enable math rendering in standard markdown preview

[mjbvz]

Feature request

We now ship support for rendering math equations inside of markdown for notebooks. However this is currently disabled for normal markdown source code

It would be great to enable this rendering for the normal markdown preview too. A few notes:

• There needs to be a simple way to turn off this feature
• We also need a bit more testing of the rendering in insiders to make sure it doesn't cause any weird issues (specifically we want to make sure markdown with $ that should not be part of a math equation doesn't do weird things)

[rzhao271]

@mjbvz can you provide some verification steps for this issue?
Using the experimental iframe, the results seem off:

[mjbvz]

Try grabbing an equation from the katex docs:

$$
c = \pm\sqrt{a^2 + b^2}
$$

Should be working in the next insiders (Thursday)

[weinand]

Works fine for me:

[Yue-Zhengyuan]

Will VS Code natively support KaTeX custom macros (see this documentation) in near future? (Although there exists 3rd party extensions like this one providing this feature, but currently they cannot work with the built-in math rendering of VS Code, i.e. I need to use KaTeX provided by these extensions instead of the built-in one.)