PHP 8.1 support

[SpacePossum]

• Never type PHP8.1 - type "never" support #5967
• Readonly properties PHP8.1 - "readonly" property modifier support #5968
• Enums PHP8.1 - Enum (start) #5978 (first small changes only to prevent conflicts)
• Array unpacking with string keys -> no syntax change
• New in initializers PHP 8.1: add new_in_initializers to PHP 8.1 integration test #6146
• Pure intersection types
• First-class callable PHP8.1 - FirstClassCallable #5972
• Final class constants PHP8.1 - "final const" support  #5973
• Explicit octal integer literal notation PHP8.1 - Explicit octal integer literal notation #5971

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• braces need checking
• ClassyAnalyzer::isClassyInvocation needs checking

[coveralls]

Coverage remained the same at 92.832% when pulling b0edbe0 on SpacePossum:master_81_never into ef44dce on FriendsOfPHP:master.

[alexander-schranz]

I tested PHP 8.1 and did got an unexepcted change which behaves different to php 8.0 or lower versions: #5993. I tested this branch and the master branch the issue there still exist. I sadly could not find out what change in PHP 8.1 does produce this issue.

• PHP 8.1 by reference has space before

[SpacePossum]

The issue you see is the result of PHP8.1 tokenizing code differently than before, especially the & has changed for which we have no fix/support ready.

[GrahamCampbell]

Some work on that was started here: #5863.