Use package extensions

[schillic]

The only noticable change is that similar_type now creates a conflict when loading both this package and StaticArrays. At least I do not see how to work around this with package extensions. But maybe that is fair. In fact, the problem existed before; there was just no direct warning when using both packages, but the warning appeared when trying to access the symbol:

julia> using ReachabilityBase.Arrays

julia> using StaticArrays  # no warning here

julia> similar_type
ERROR: UndefVarError: `similar_type` not defined in `Main`
Hint: It looks like two or more modules export different bindings with this name, resulting in ambiguity. Try explicitly importing it from a particular module, or qualifying the name with the module it should come from.
Hint: a global variable of this name also exists in StaticArrays.