[WIP] Add basic support for PHP 8 union types

[wouterj]

I noticed that running Psalm on PHP 8 results in an error in my build:

Uncaught Error: Call to undefined method ReflectionUnionType::getName() in /home/runner/work/WouterJEloquentBundle/WouterJEloquentBundle/vendor/vimeo/psalm/src/Psalm/Internal/Codebase/Reflection.php:415

The patch in this PR fixes the build, but more work needs to be done. (I'm also unsure about the policy of this package to unstable PHP versions)

I'm very very new with the Psalm codebase, so I'm unsure how to continue. I'm happy to finish this PR if there is some guidance, but also please feel free to take over this PR if that's easier for you :) A list of things I'm stuck with atm:

• Psalm doesn't yet know about the ReflectionUnionType class, so the analysis errors. I found stubs/Php80.php and dictionaries/CallMap_80_deta.php. Am I supposed this add that class somewhere? (I've tried doing so, but it didn't seem to do much) For reference, this is the ReflectionUnionType structure: https://wiki.php.net/rfc/union_types_v2#reflection
• Psalm incorrectly thinks getName() is part of ReflectionType, it's part of ReflectionNamedType instead. I couldn't find a way to tell Psalm this.
• I also get another Psalm error with this patch: Could not verify return type 'Psalm\Type\Union' for Psalm\Internal\Codebase\Reflection::getPsalmTypeFromReflectionType I'm unsure why that is, as this method is always returning Type\Union as far as I can see.
• This surely needs a test. I couldn't find tests directly for Reflection or php8. Can you guide my to which test class I should add a new test to cover this?

[muglug]

I added a stub for ReflectionUnionType in 114440c

[muglug]

Thanks, this is good, I'll fix any remaining errors in master.

[wouterj]

Thank you. I can confirm this has fixed the "bug" in my build: https://github.com/wouterj/WouterJEloquentBundle/runs/1371434654

Btw, I just noticed that getTypes() returns list<ReflectionType>, instead of the typehinted ReflectionNamedType in the array_map function. Maybe the code needs to be adapted to recursively transform either ReflectionNamedType or ReflectionUnionType to a parseable string?