Plugin Support for Custom Transformers

Since #13764 has landed, it's easy to write custom transformers. However, if I understand the API correctly, it is needed to duplicate the whole tsc command line just to add a single transformer. This leads to incompatibilities since these, on typescript depending, applications do not support the same features as the tsc command line tool.

It, therefore, would be favored to have a plugin system that allows loading Custom Transformers from third party node modules as this is the case for the language service proxies #12231. These transformers then easily integrate into existing build tools / build workflows.

If someone experienced has inputs on how to implement the changes, I'm willing to create a PR as it would simplify my project tremendously.