fix: Adjust math delimiter to fix markdown import/export compatibility issue

[dreamerblue]

This PR is expected to fix the markdown import/export compatibility issue in #6650.

Test cases

Case 1: Import

Here is the test case, save as .md and use it to import document:

## Inline

Here is inline $x=y$

## Block

$$
y=x
$$

Excepted result in Outline editor:

All general LaTeX texts should be converted to Outline math_inline or math_block.

Case 2: Input and view in editor

This PR won't modify the input shortcut and trigger key. So the original trigger points ($$ for inline, $$$ + Space for block) are preserved. And all imported or manaually inputed math nodes is still correct in state, only the generated text is changed.

Case 3: Export

Export an Outline document which contains math inline/block to markdown.

The result .md shoule be like (All formulas can be rendered correctly in any other editors):

Other notes

I'm new to contributing to this project, and after reading some of the source code and trying to modify it, I verified these test cases above locally.

This modification does not automatically fix documents that already have custom text cached in the Outline database (e.g. $$$), it only works for new incremental documents. Perhaps the core maintainer could help add a batch function for auto-fixing. The old text cache may only be affected if the export function is used. Viewing and editing in Outline still works fine for older documents.

[CLAassistant]

All committers have signed the CLA.