Port --enforceReadonly to the Go compiler

[Copilot]

Ports the behavioral changes from microsoft/TypeScript#59326.

• Adds enforceReadonly compiler-option parsing, CLI/help/init output, diagnostics, and API schema generation.
• Rejects readonly-to-mutable property and index-signature relationships outside comparable relations; preserves method exemptions.
• Extends mapped-type relation handling and const-context inference for readonly constraints.
• Adds compiler and command-line baseline coverage.

// @enforceReadonly: true
let mutable: { x: string };
declare const readonlyValue: { readonly x: string };

mutable = readonlyValue; // Error