chore: enhance type to keep reference to its T

[yusuke99]

Closes #258

When creating a stub type from a server class T, it drop keys such as brand symbols. Currently it iterates with Exclude<keyof T, …>, which makes TypeScript then treats the result as a new object, so "Go to Definition" on api.authenticate() no longer jumps to T.authenticate. This PR still drops the same keys, but maps over keyof T first and then Pick.

Here is a minimal PoC difference between using Exclude<keyof T, …> vs using Pick:

https://www.typescriptlang.org/play/?#code/MYGwhgzhAEDKYFsAOICm0DeAoa0BmA9gQBQCUmOu0ATqgC4Cu1AdtHdQ6gNyUC+lAIzDUyFKjXpNW7Tj1z9+WOgE8k6AIIAeACoA+aAF4x0ANoBpaAEtWAUQAeoBgBNUmgNaplBPNG0AaaAByIWpA3QBdAC5fc3CeXh4lVXQAIR19IwAFS2A3TUpscXMrVg8vH20omLM4vj9Ke0cXd09vXwDg4TCsXUSXUGF0YAJmCDpoMGiteGQ0Xqx+8FpoYdHxgWi0mZRUeawwADpCElIeAHoz6AA9AH4sASOiMnPL26wsIA

capnweb.mp4

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[yusuke99]

I have read the CLA Document and I hereby sign the CLA

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commit: 19c2bf4

[ndisidore]

Thanks for the DevEx wins and the included video! That's how you do a PR right there.
Looks good, just getting a few others on the team to take a look