feat: Add header comment generator configuration option

[ascott18]

Motivation

Generated code can be confusing without clear markers indicating its origin. This PR adds a configurable HeaderComment option that allows developers to emit informative comments at the start of generated files, helping prevent accidental edits and documenting the generation source.

Changes

• New HeaderComment property on the Generator base class with [GeneratorConfig] attribute
• Cascading config support in CompositeGenerator so child generators inherit parent header comments unless they define their own
• Implementation in StringBuilderFileGenerator to prepend formatted comments to generated output
• Default configurations in coalesce-vue3.json for the playground and CreateVitestTargets test
• .prettierignore file added to repo root to ignore generated *.g.ts and *.g.tsx files

Usage

Configure header comments in coalesce.json:

{
  "generatorConfig": {
    "Scripts": {
      "headerComment": "Generated code - do not edit directly. Generated with: pnpm coalesce"
    }
  }
}

Child generators (TsMetadata, TsModels, etc.) automatically inherit the parent's header comment through the cascading system.

Generated Output

Generated TypeScript files now include a header comment for clarity:

// Generated code - do not edit directly. Generated with: pnpm coalesce

import { ... }

Notes

• The feature uses simple // comment prefix since Coalesce only generates TypeScript, JavaScript, and C# files
• Header comments are placed after ESLint/Prettier directives (if added in the future)
• All tests pass; CreateVitestTargets test validates the feature