support alias definitions in package.json

[schlichtanders]

I am trying to get importmaps running for preact in combination with @mui/material (which needs react to be aliased to preact/compat and similar).

While importly is the only import map generator so far which does not fail creating an importmap on nodemodules, it unfortunately does not respect the alias mapping. React is not pointed towards preact/compat...

Please advise on how to set aliases, as preact would really be awesome in such an importmap setup