feat: Add support for HJSON

[polarathene]

Just adding a note here for reference:

• HJSON support was removed in the past: https://github.com/mehcode/config-rs/issues/198
• Recently that crate upgraded to Serde 1.x: https://github.com/hjson/hjson-rust/issues/6#issuecomment-2245155197

If I ever have the time to wrap up my own PRs for config-rs, adding back hjson support would be very easy now.

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Waiting on new release: https://github.com/hjson/hjson-rust/issues/25

[epage]

Note about adding a new format: traditionally, we've enabled all of them by default but #621 would have us re-evaluate that assumption.