Add custom LaTeX delimiters to preferences

[Paalon]

See #1201. This add optional LaTeX delimiters.


However, currently this has a bug that LaTeX preference value seems not to be saved but I cannot understand. I want help to solve.

[sosukesuzuki]

Please fix the commented place. 🙏
And, is yarn.lock necessary? Please delete it if you do not need it. 👍

[sosukesuzuki]

Please use const instead of var.
In a modern browser, please use let or const.

[Paalon]

The bug was fixed. That was only my mistake of property names.

[sosukesuzuki]

Thanks for your fixing! 👍

[Paalon]

There are something to note.

1. The change of delimiters seems not reflected unless restarting.
2. I set the default delimiters {inline: $, block: $$}, but the current master version's default setting is {inline: $$, block: $$$} because it uses default setting of @rokt33r/markdown-it-math. I don't know what delimiters are most used (e.g. Original LaTeX: {inline: $ and $ or \( and \), block: $$ and $$ or \[ and \] or \begin{something} and \end{somthing}}, KaTeX, Typora, Quiver default: {inline: $, block: $$}, MacDown default: {inline: \( and \), block: $$}, @rokt33r/markdown-it-math's default: {inline: $$, block: $$$}). I like {inline: $, block: $$}, but someone will prefer others.