feat: Add an is_public helper method to guess if an object is public

A `public` attribute is also added to Object and Alias,
allowing extension writers to force an object to be public.

src/griffe/dataclasses.py

@@ -334,6 +334,7 @@ def __init__(
         self.aliases: dict[str, Alias] = {}
         self.runtime: bool = runtime
         self.extra: dict[str, dict[str, Any]] = defaultdict(dict)
+        self.public: bool | None = None
         self._lines_collection: LinesCollection | None = lines_collection
         self._modules_collection: ModulesCollection | None = modules_collection
 
@@ -761,6 +762,7 @@ def __init__(
         self.alias_endlineno: int | None = endlineno
         self.runtime: bool = runtime
         self.inherited: bool = inherited
+        self.public: bool | None = None
         self._parent: Module | Class | Alias | None = parent
         self._passed_through: bool = False
         if isinstance(target, str):

src/griffe/mixins.py

@@ -293,6 +293,39 @@ def is_implicitely_exported(self) -> bool:
         """
         return self.parent.exports is None  # type: ignore[attr-defined]
 
+    def is_public(
+        self,
+        *,
+        strict: bool = False,
+        check_name: bool = True,
+    ) -> bool:
+        """Whether this object is considered public.
+
+        In modules, developers can mark objects as public thanks to the `__all__` variable.
+        In classes however, there is no convention or standard to do so.
+
+        Therefore, to decide whether an object is public, we follow this algorithm:
+
+        - If the object's `public` attribute is set (boolean), return its value.
+        - In strict mode, the object is public only if it is explicitely exported (listed in `__all__`).
+            Strict mode should only be used for module members.
+        - Otherwise, if name checks are enabled, the object is private if its name starts with an underscore.
+        - Otherwise, if the object is an alias, and is neither inherited from a base class,
+            nor a member of a parent alias, it is not public.
+        - Otherwise, the object is public.
+        """
+        if self.public is not None:  # type: ignore[attr-defined]
+            return self.public  # type: ignore[attr-defined]
+        if self.is_explicitely_exported:
+            return True
+        if strict:
+            return False
+        if check_name and self.name.startswith("_"):  # type: ignore[attr-defined]
+            return False
+        if self.is_alias and not (self.inherited or self.parent.is_alias):  # type: ignore[attr-defined]
+            return False
+        return True
+
 
 class SerializationMixin:
     """A mixin that adds de/serialization conveniences."""