Load mathjs in a browser module (ES import)

Currently it looks like it's not possible to use mathjs in a browser module like:

```html

import { create, all } from 'mathjs'

```

I think the existing bundles in `dist` do not have the right exports to be consumed as an ES module.

What should work (but doesn't) is loading like:

```
import { create, all} from 'mathjs/main/es/index.js'
```

I think the reason is that many import paths in the code are missing the explicit "*.js" extension, which is the normal way to go in nodejs and webpack environments, but not in browser ES environments which require a real path to a file.