Add compiler support up to Delphi 12

[delphidabbler]

Add missing compiler support up to Delphi 12 Athens.

Keys have already been defined in CodeSnip 4.22.0.

[delphidabbler]

Added documentation for new compiler keys for Delphi 10.2 to 11.x as at commit cf326b1.

Still to document Delphi 12.x

[delphidabbler]

This issue replaces (and includes) issue #2 (Add compiler support up to Delphi 11).

[delphidabbler]

Support for Delphi 12 added to documentation at commit 7ed2037.

Except for new snippets added when implementing issues #20 and #25, no snippets have had compile results added for Delphi 11 or 12. This is a big job and a separate issue (#27) has been created for that job.

This issue is now deemed to be completed.