Enhancement: Placeholder support for slugs

[estruyf]

Like the preview path, the slug should also be able to use placeholders: https://frontmatter.codes/docs/content-creation/placeholders - that way, users can use any fields to include their slugs.

The configuration should be on the global level and content type level.

[estruyf]

To be tested after the change:

• Placeholders on fields
• File creation with placeholder
• Update slug command
• Preview
• Snippets in the snippets dashboard (placeholders on fields)

[estruyf]

New settings:

{
  "frontMatter.taxonomy.slugTemplate": "{{title}}",
  "frontMatter.taxonomy.contentTypes": [
    {
      "name": "default",
      "pageBundle": false,
      "slugTemplate": "{{fm.pubDate|format:yyyy-MM}}-{{title}}",
      "fields": []
    }
  ]  
}