Markdown katex macro

[ziqiq]

This patch enables users to define page level and global katex macro, so that they don't have to type long but frequent math expression over and over again.

Example:

Global macros in pagic.config.ts

export default {
  md: {
    katexMacros: {
      "\\Z": "\\mathbb{Z}",
      "\\Null": "\\mathit{Null}",
      "\\cycl": "\\langle #1 \\rangle"
    },
  },
};

Some additional macros in index.md

$$
\gdef \greetings#1{\text{Welcome to my site, #1!}}
\gdef \degree{^\circ}
$$

$$\greetings{World}$$

A $90 \degree$ counter-clockwise rotation.

The null space of a matrix is $\Null(A) = \{\vec{x}\in \R^n : A\vec{x}=\vec{0}\}$

A cyclic subgroup is $\cycl{g} = \{ g^k | k \in \Z \}$.

result: