External Symbols are lacking cleanup

[Josef Kučera]

Hello, I have discovered that when using more instances of TDelphiWebScript (some of them short-lived) the ExternalVar Symbols are not unregistered from the TExternalSymbolHandler causing bloat and potential trouble (e.g. after unloading a dynamic library with a Handler code used for specific scripting). I have added the Unregister methods and it seems to be working fine right now. The patch is attached.

Best regards, Joe