[#204] Fix the Indigo tutorial REPL command

Description

Problem: Current stack ghci indigo causes ambiguous error Ambiguous occurrence ‘>>=’ . A workaround is to load the file directly stack ghci code/indigo/src/Indigo.hs, but that is not ideal.

Solution: By using stack target syntax, we can add an executable component inside indigo, and only load that executable via stack ghci indigo:indigo-repl or stack ghci :indigo-repl.

Related issue(s)

Resolves #204 (closed)

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