[#60] add Indigo's ifThenElse definition for RebindableSyntax

Description

This adds an ifThenElse function definition to Indigo (which uses RebindableSyntax) to be used in the if...then...else construct.

This is implemented so that the condition exa respects the constraint IsExpr exa Bool.

Related issue(s)

Resolves #60 (closed)

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