proposal: Remove vague utility types and migrate to System.Text.Json

[alsi-lawr]

Remove vague *Utils types, and migrate from Newtonsoft.JSON -> System.Text.Json.

Motivation

Types like StringUtil are made redundant by adopting System.Text.Json. The EnumUtil should be renamed to better encapsulate what it's actually responsible for.

Scope

• Replace StringUtil with JsonNamingPolicy from STJ.
• Replace Newtonsoft.Json with STJ, or drop entirely if not needed.
• Rename vague *Utils types into purpose-specific types.
• Keep changes focused and behaviour-preserving where possible.

Acceptance criteria

• No vague utility type remains for identifier naming.
• Newtonsoft.Json is replaced with STJ.

Blocked by

• proposal: Add enforced CSharpier formatting #27