Add SIXX to Object Serialization

[StewMacLean]

I have found SIXX useful for porting objects between dialects of Smalltalk.

Suggest adding https://github.com/mumez/SIXX

to

https://github.com/pharo-open-documentation/pharo-wiki/blob/master/PharoProjects/ObjectsSerialization.md.

SIXX has problems loading in Pharo 9 due to missing GLMHighlighterTextStylerDecorator, but just ignore the walkback on load and continue. Basic serialisation still works.