TOML is enabled by default. Enable json5 or yaml when users already work in
that format. YAML uses YAML 1.2 semantics in v0.9.0: legacy words such as yes
and on remain strings, and duplicate mapping keys are rejected.
enabled: yes
mode: on
port: 8080
Enable YAML with features = ["yaml"]; this also requires the serde_json
feature, which is part of the default feature set. If defaults are disabled,
enable both explicitly. Treat a change from v0.8.0 YAML parsing as a data
migration and run representative production files through v0.9.0 before
deploying.