Use `fieldRename: .snake` instead of hard coding field names on json

[jtmcdole]

https://pub.dev/documentation/json_annotation/latest/json_annotation/FieldRename.html

skills/tool/dart_skills_lint/lib/src/models/ignore_entry.dart

Line 21 in 9eff6bb

static const String ruleIdKey = 'rule_id';