add type checking

[irahopkinson]

• use noImplicitReturns instead of consistent-return

Note this is draft because I had to comment out 3 lines in tsconfig.json in order to get the type checking passing. We don't strictly need to have it passing here since we don't check it on commit however having it failing might not be great either and the commented lines are either removed or modified in core anyway.

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This change is