Fix the standalone release tarball; update compiler list and CITATION

Squashed onto develop from:
- Make the json.tar.xz release archive configure standalone
- Update the supported compiler list and the CITATION.cff repository link
- Add json_fwd.hpp to the Bazel singleheader-json target
- Document SwiftPM's #include <json.hpp> form and add CI coverage
- Migrate REUSE metadata from deprecated .reuse/dep5 to REUSE.toml

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

CITATION.cff

@@ -10,5 +10,5 @@ title: "JSON for Modern C++"
 version: 3.12.0
 date-released: 2025-04-07
 license: MIT
-repository-code: "https://github.com/nlohmann"
+repository-code: "https://github.com/nlohmann/json"
 url: https://json.nlohmann.me