Improve Squeak 6 Support

[marschall]

• move methods from GRPharoPlatform to GRSqeuakPlatform
• keep GRPharoPlatform as empty
• migrate from FileSystem to FileDirectory
• implement missing methods
• custom implementation of #openDebuggerOn:
• add test for deprecationExceptionSet

[codecov]

Codecov Report

Attention: Patch coverage is 48.21429% with 29 lines in your changes missing coverage. Please review.

Project coverage is 85.55%. Comparing base (737de76) to head (4c94ee7).
Report is 1 commits behind head on master.

Files	Patch %	Lines
...form.class/instance/readFileStreamOn.do.binary..st	0.00%	13 Missing ⚠️
...GRSqueakPlatform.class/instance/classCommentOf..st	0.00%	7 Missing ⚠️
...GRSqueakPlatform.class/instance/openDebuggerOn..st	0.00%	5 Missing ⚠️
...package/GRSqueakPlatform.class/class/initialize.st	0.00%	2 Missing ⚠️
...latform.class/instance/ensureExistenceOfFolder..st	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
+ Coverage   85.12%   85.55%   +0.43%     
==========================================
  Files        2054     2046       -8     
  Lines       14121    14072      -49     
==========================================
+ Hits        12020    12039      +19     
+ Misses       2101     2033      -68     

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[marschall]

I have no idea why our Squeak 6 CI tests are green. They fail when I load them into an image. There were two main issues:

• The code was relying on FileSystem instead of FileDirectory but FileSystem is not present by default. Maybe Smalltalk CI did load it.
• GRSqueakPlatform was extending GRPharoPlatform, there is a 50:50 chance which one would be active at any time

This change contains two main changes

• Move all methods from GRPharoPlatform to GRSqeuakPlatform but keep GRPharoPlatform as an empty superclass for this. The reason for this several packages lack a -Squeak version so we load the -Pharo version which defines extension methods on GRPharoPlatform. This should keep that working.
• Implement missing methods and address all Grease Squeak related issues known to me.

[krono]

Thanks for taking care!