Detect template placeholders and set dummy values if not provided by the user, allowing easier use for many basic cases. Prints warnings when dummy values are used.
Override some gojsonscheme error strings with more helpful messages.
Skip noisy root error ("must validate one and only one schema").
Replace ghodss/yaml with goccy/go-yaml. The latter is more maintained and provides better error messages for invalid YAML inputs.
Fixes #321.