Build should fail when using an unknown function

[Bastien Botella]

Is your feature request related to a problem? Please describe.
What using a native CSML function, in a flow, build should fail if this function is neither in the current flow nor imported. This would avoid errors in runtime.

Describe the solution you'd like
The following flow should fail to build

start:
  say aRandomFunction()
  goto end

The code above should build only if the function in present in the flow or imported

Describe alternatives you've considered
None