use vscode-markdown-it-katex to render kaTex

[ZekeLu]

1. it bundles the extension with esbuild to reduce the size of the extension (from 9.8M to 1.5M).
2. it renders KaTex with @vscode/markdown-it-katex which is the same one used by VS Code.

Fixes #17, #21, #167, #355. And it's an alternative to #198.

Anyone who wants to try it, download the extension from https://github.com/ZekeLu/vscode-markdown-pdf/releases/tag/1.5.1 and install it manually.