Support declarations generation after transform

[zsilbi]

Requires #34

Output files coming from these mkdist loaders that are marked as declaration: true will be set to declaration: "generate" so we can collect them before writing the output and run the declarations emit altogether.

Originally posted by @zsilbi in #34 (comment)

For Vue SFC:

i think we can have the fallback transformer as mkdist did, since vue-tsc is not support allowArbitraryExtensions yet.

Originally posted by @Teages in #34 (comment)

[pi0]

Thanks for issue. Lets keep tracking in same #34 (mainly to ensure initial version considers all needs like this)