Add Document-Level Modifiers

[butschster]

This PR introduces the ability to define modifiers at the document level that apply to all compatible sources within that document.

Problem

Previously, modifiers could only be defined at the source level, requiring repetition when the same modifier needed to be applied to multiple sources within a document.

Solution

Implemented an immutable ModifiersApplier system that:

• Allows documents to have their own set of modifiers
• Passes these modifiers to all source fetchers during compilation
• Combines document-level and source-level modifiers when applicable
• Applies modifiers to content after fetching

Example Usage

{
  "documents": [
    {
      "description": "API Documentation",
      "outputPath": "docs/api.md",
      "modifiers": ["php-signature"],
      "sources": [
        {
          "type": "file",
          "sourcePaths": ["src/Api"],
          "filePattern": "*.php"
        }
      ]
    }
  ]
}

---

Add support for modifier aliases in configuration

This PR adds support for defining and using modifier aliases in the configuration. This feature allows users to define common modifier configurations once and reference them by name, reducing duplication in the configuration.

Problem

Currently, users need to repeat the same modifier configurations across multiple documents or sources, leading to duplication and maintenance challenges.n

Example Usage

Users can now define modifier aliases in their configuration:

{
  "settings": {
    "modifiers": {
      "api-docs": {
        "name": "php-content-filter",
        "options": {
          "keep_doc_comments": false,
          "keep_method_bodies": false
        }
      }
    }
  },
  "documents": [
    {
      "description": "API Documentation",
      "outputPath": "docs/api.md",
      "sources": [
        {
          "type": "file",
          "sourcePaths": ["src/Api"],
          "modifiers": ["api-docs"]
        }
      ]
    }
  ]
}