Simplify bin creation

Currently we manually have to create useless proxies like these: https://github.com/kevva/elm-platform-bin/tree/master/bin

The reason is that we have to define the path to the binary in package.json up front.

Another way we could do it is to instead have `bin-wrapper` copy/move the binary to a fixed path/name we define in package.json.

@kevva What do you think?

Start by reviewing the proxy examples in the linked elm-platform-bin/bin directory and how package.json currently defines binary paths. Clarify the fixed path/name behavior and its compatibility with the existing package setup before proposing the change; the issue does not identify tests or a completion check.

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