Feature(transformer): re-implement postProcess API

[Shinyaigeek]

↪️ Pull Request

Part of #7503

postProcess API was removed in #5674 because this API was unused, but this API is necessary for svelte preprocessing, so I start by re-implement this postProcess API in order to accomplish no-config svelte support.

🚨 Test instructions

Transformer’s API, such as transformer’s generate method, does not seems to be tested directly and a testing method does not seem to be established. In addition to these reasons, I thought Transformer’s API is tested indirectly by much of integration-test, so I did not write any unit-test about this API, but I think it would be better to write the unit-test about postProcess, so I will write a test if you want.

✔️ PR Todo

• Added/updated unit tests for this change
• Filled out test instructions (In case there aren't any unit tests)
• Included links to related issues/PRs

[Shinyaigeek]

seems to be a change by the formatter, which runs pre-commit

[Shinyaigeek]

other than config type on this line and some comments and some invalidation logic are completely same to https://github.com/parcel-bundler/parcel/pull/5674/files

[devongovett]

Once we have something that uses this, we should add some caching tests for this. One possible issue is if a transformer plugin returns a different list of assets from postProcess, the invalidations/dependencies in the original assets will be lost resulting in the cache/watcher not being invalidated on changes. Not sure if we should/can do this automatically in core, or whether we need to leave that to plugins. We'll see as this progresses further.