Support preamble

I added a macro to the specification file

```typst
...
    LoadClassWithOptions: ignore-sym,
    // user-defined
    Li: sym,
  ),
)
...
```

but it didn't work

```typst
#import "@preview/mitex:0.2.4": *

#mi("\\Li_2(x)")
```

```
plugin errored with: error: unknown command: \Li
```