Enable "Menu: Main navigation" in the Sitemap by default

Great issue! This was not possible with recipes when this recipe was created. Since the Sitemap module keeps the plugins in its sitemap.settings.yml, we were not able to change that file and continue to be idempotent.

With the introduction of config:strict in 10.4, we now can.

We want:

page_title: Sitemap
path: /sitemap
message:
  value: ''
  format: plain_text
plugins:
  frontpage:
    base_plugin: frontpage
    enabled: true
    weight: 0
    settings:
      title: 'Front page'
      rss: /rss.xml
    id: frontpage
    provider: sitemap
  'menu:main':
    base_plugin: menu
    enabled: true
    weight: 0
    settings:
      title: 'Main navigation'
      show_disabled: false
    id: 'menu:main'
    provider: sitemap
include_css: true

Instead of:

page_title: 'Sitemap'
path: 'sitemap'
message:
  format: 'plain_text'
  value: ''
plugins: []
include_css: true

MR added.

One minor stylistic question. This would probably also benefit from a very simple test of some kind, just to prevent it from regressing in the future.