[12.x] Improve Collection type inference for keys() and all() methods

[kayw-geek]

Description

Improves static analysis type inference for Collection keys() and all() methods.

Changes:

• keys(): Return type from Collection<int, TKey> to Collection<never, TKey>
• all(): Return type to list<TValue> when TKey is never

Before:

/** @var list<int> $list */ // Manual annotation required
$list = $collection->keys()->all();
array_is_list($list);

After:

$list = $collection->keys()->all(); // Automatically inferred as list<int>
array_is_list($list);

[shaedrich]

Does never make sense here? Isn't TKey always array-key which can be narrowed down to either string or int if possible by the underlying data? 🤨