Add basic_json::as_base_class and document name conflicts with custom base classes (#5589)

* Add basic_json::as_base_class and document name conflicts with custom base classes

Members of basic_json hide members of a custom base class with the same
name, and future releases may add members that hide ones accessible
today. Document this in json_base_class_t and add as_base_class() to
reach hidden members without spelling out the cast.

Also make json_base_class_t a public member type. It was documented
since 3.12.0, but declared private, so users could not name it.

Supersedes #3899.

Co-authored-by: Raphael Grimm <1005058+barcode@users.noreply.github.com>
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Add as_base_class to the docset search index

New public members get an entry in docs/docset/docSet.sql (as done for
to_bon8/from_bon8 in #2998). Without it, the Dash/Zeal docset built from
the documentation cannot find basic_json::as_base_class.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Silence clang-tidy for the hidden type_name() in the base class test

ci_clang_tidy failed with readability-convert-member-functions-to-static
on base_class_with_hidden_members::type_name(). It must stay a
non-static member: the test shows that it is hidden by the non-static
basic_json::type_name() and reachable through as_base_class().

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Co-authored-by: Raphael Grimm <1005058+barcode@users.noreply.github.com>

docs/mkdocs/docs/api/basic_json/index.md

@@ -200,6 +200,7 @@ Direct access to the stored value of a JSON value.
 - [**get_ref**](get_ref.md) - get a reference value
 - [**operator ValueType**](operator_ValueType.md) - get a value
 - [**get_binary**](get_binary.md) - get a binary value
+- [**as_base_class**](as_base_class.md) - access the custom base class
 
 ### Element access