Improve type definitions for 'mathjs/number'

Hi,

I tried following the directions of the webpage regarding custom bundling ( https://mathjs.org/docs/custom_bundling.html ), but it seems like there is an issue with using the sub-modules when in a TypeScript project.

The error I get when trying to import mathjs/number as described in the article is as follows:
`import { create, all } from "mathjs/number";`
`Cannot find module 'mathjs/number' or its corresponding type declarations.`

In my package.json, I have version 9.4.3 of mathjs, and 9.4.2 of @types/mathjs. I checked the installed mathjs in my node_modules, and its package.json does indeed export './number', so I'm confused as to why it's not being picked up. Could it be that type definitions are not being imported along with it, therefore throwing an error?

Maybe adding something like `"mathjs/number": ["./index.d.ts"]` to `paths` of mathjs's package.json could help? (It doesn't, but maybe something similar does)