Support php 8.1 enums (#197)

* Support php 8.1 enums

* Test with travis and php 8.0, again

* AST_ENUM_CASE can have attributes

* Set `type` property of `AST_CLASS`

Support php 8.1 enums (#197)

* Support php 8.1 enums

* Test with travis and php 8.0, again

* AST_ENUM_CASE can have attributes

* Set `type` property of `AST_CLASS` iff ast version 85+

(iff === if and only if)

* Add AST_CLASS's type in version 85+ prior to php 8.1

* [skip ci] Document support for php 8.1 enums

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