Change return type of generated setters to the message type

[craigatron]

Changes

• message.ts - changed return type of generated setters to ${messageName}
• ran npm run generate to regenerate example proto generated code
• added tests that chain setter calls and verify value on generated object

Verification

Added unit tests for chained setter methods.

[moadz]

From the Protoc docs:

addFoo(): Appends a value of foo to the end of the list of foos that was in the message. Returns the outer message for chaining only if the added value was a primitive. For added messages, returns the message that was added (b/138079947).

https://developers.google.com/protocol-buffers/docs/reference/javascript-generated

My understanding is this means the return type of setInternalChildMessage for example should be the child message type, and not the parent message type.

        // Wrong 
        setInternalChildMessage(value?: OneOfMessage.InternalChildMessage): OneOfMessage;
        
        // Correct
        setInternalChildMessage(value?: OneOfMessage.InternalChildMessage): OneOfMessage.InternalChildMessage;

Can you add some test cases to verify this?

[gecko655]

This fix is seriously important for the ts-protoc-gen users (and me).
It seems that fixes and tests are good enough (for me).
Would it be possible for the maintainers to review again?

Note: This PR fixes #218 #285 .

Update:
This PR also fixes #289

[improbable-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[chris-codaio]

Just ran into this issue trying to update definitions. It looks like this PR is approved - any reason it can't be checked in and pushed out in a new version? Anything I can do to help?

[thompsonja]

Bumping this PR... is there any reason this can't be merged?

[gonzojive]

bump