Fix block action detection, remove StringIndexed from action body types

[aoberoi]

Summary

The issue is described in much more detail here. This PR does two things:

1. Removes StringIndexed from SlackAction (and the interfaces in that union). SlackAction represents the body of an action request, and in this case the fact that each of its constituent types also had an index signature (provided by StringIndexed) meant that the typechecker allowed unknown property names to evaluate to any. This is working against us when we use types to describe how we manipulate or conditionally check these action bodies. We might want to remove StringIndexed from more places in the future.

2. Changes the action property lookup to actions. This fixes the issue with detection.

Fixes #163

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