JSON5 conf files

[sim642]

Yojson now has JSON5 support: ocaml-community/yojson#152. This would give support for comments and trailing commas.
The annoying thing about JSON5 is that Microsoft and VSCode seem to be immune to it (microsoft/vscode#100688), so JSON5 syntax highlighting requires installing a separate extension.

Actually, Yojson's non-JSON5 parser also allows comments which we could already use, but that might confuse some other tooling. For example, VSCode doesn't like comments in .json files, but is fine with them using .jsonc extension (for JSON with comments). But that extension is quite uncommon.