Export FScanner parser to ZScript as ScriptScanner

[Cacodemon345]

This PR exposes the internal FScanner parser to ZScript as ScriptScanner.

ScriptScanners are created with Create functions, and destroyed by calling Destroy (after which it must not be used). SetPrependMessage is used to set the message that will be prepended to the displayed error messages in case of failure. The rest of the exported functions work unchanged from their native counterparts.

[coelckers]

Please rewrite this to not use multiple inheritance. A lot of ZScripts inner workings circumvent standard C++ logic that ensure type safety and this may easily end up trashing the pointer to such an object because it may not point to where you expect it anymore.

[Cacodemon345]

Done as requested.

[nashmuhandes]

Please also provide a working example mod that showcases the usage of this feature.

[Cacodemon345]

scriptparserexample.zip
demoscpt.txt

Here's one. Compile and run this PR with the ZIP file and the uploaded demoscpt.txt lump and open any kind of map. It will print 1337 to console.

demoscpt.txt

Since this example uses MustGetStringName function, it will abort to the console with the second one.

[coelckers]

I merged this manually and changed it to use direct native calls for most functions