Install DocStringExtensions

Automatically generates typed signatures using [DocStringExtensions.jl](https://github.com/JuliaDocs/DocStringExtensions.jl).

```
import DocStringExtensions

"""
$(DocStringExtensions.TYPEDSIGNATURES)
"""
function hello()::String
    return "Hello, World!"
end
```

Example: https://github.com/ohno/MyPkg56.jl/blob/57e84de23e28ea8984496cd724bc56556c366927/src/MyPkg56.jl