feat: Message type guard helpers

[msfstef]

Addresses #1453

Adds the type guards to check whether a message is a ControlMessage or a ChangeMessage, documented, tested, and exported.

I've also honed in on the message type definitions a bit more as I had experienced an uncaught bug due to the generality of the Headers type.

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[KyleAMathews]

great! These will be very helpful.

Could you check the examples for where these would be helpful as well? Anywhere using ShapeStream. E.g. the Redis example needs it

electric/examples/redis-client/src/index.ts

Line 24 in b272d3f

if (!(`key` in message)) return

[kevin-dp]

Great!
Is there a reason we have packages/typescript-client/test/helpers.test.ts and packages/typescript-client/test/helpers.test-d.ts ? These are executing different tests, can we merge them into one file?

[msfstef]

@KyleAMathews will update codebase in this same PR to use these helpers

@kevin-dp the test-d.ts files are "type tests" (see vitest docs) which essentially are making sure the type system is behaving as intended. In order for the test to test types rather than test code (and separately have tsc for the typecheck of the test files), I like having the separate "type tests" that ensure our type system makes sense explicitly.