missing A3 pagesize

this is simply fixed by adding: 
```
export const PAGE_SIZE_A3 = 'axidraw-web-page-size-a3';
```
and
```
  {
    type: PAGE_SIZE_A3,
    alias: 'A3 (420mm x 297mm)',
    width: 420,
    height: 297,
    defaultPadding: 15,
  },
```

to the PageSize in page.ts. I'd do it myself, but I cannot put in a PR to this repository :)

