Goatfile Pre-Script

[zekroTJA]

It should be possible to add a "Pre Script" to a Goatfile, whichs acts as a script section
idependent from requests, which is executed before the Goatfile actions are executed.

This could be useful to initialize variables with default values or create utility functions*.

This should be implemented with or after #44 has been implemented because this would fit nicely in a metadata header of Goatfiles.

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*Currently, function declarations are not carried over between the states of each action. Therefore,
this would have to be adjusted to make this possible.