More build outputs for package command

[dummdidumm]

Describe the problem

The package command provides a good solution already for outputting raw Svelte files along with type definitions. People wanting to provide self-contained build outputs are left out for now.

Describe the proposed solution

Provide the option to create a bundled version which is compiled an self-contained so people can use the library in non-Svelte contexts. Formats are ESM and maybe UMD.

Alternatives considered

Some docs on how to set this up yourself

Importance

nice to have

Additional Information

Continuation of #518

[ignatiusmb]

Would this be a superset of #2572?

[dummdidumm]

Damn I'm stupid, creating my own duplicate issues 😄 Closing this one as duplicate of #2572 and enhancing the issue text there a little.