Fix for #2919 option 1

[Maxim-Mazurok]

Gets rid of named exports

Potentially breaks extensions as we can no longer use this:

declare module 'mathjs' {
  interface MathJsStatic {
    testFun(): number
    value: number
  }
}

{
  const math = create(all, {})
//...

and have to use this instead:

interface MyMathJsStatic extends mathjs.MathJsStatic {
  testFun(): number
  value: number
}

{
  const math = mathjs.create(mathjs.all, {}) as MyMathJsStatic
//...

[josdejong]

See #2919 (comment)