Merge package.json "exports" with template instead of overwritting

[Goodwine]

when having a package.json file with exports like

"exports": {
  "types": "./path/to/types"
}

This currently becomes

"exports": {
  "default": "./${pkg}.default.dart.js"
}

Ideally this should become

"exports": {
  "types": "./path/to/types"
  "default": "./${pkg}.default.dart.js"
}

Note that "exports" can also be string|string[], so the feature request is to keep the behavior of overwriting the field if it was either string|string[]|undefined|null however when the field had value {} (or Map in dart) then use the spread operator and only overwrite "node", "browser", and "default" when specified.

See sass/dart-sass#1714

[Goodwine]

An alternative would have been defining a new configuration for "types" and then include it in exports but that knob becomes very fragile and it's better to just rely on merging the objects IMO