docs(json-schema): add suggested extends values

[jamietanna]

Changes

As noted in #40439, it would be useful to provide editor suggestions for
the possible values of extends, so a user can start their
configuration with recommended presets.

We can do this using a custom anyOf that includes these options as an
enum.

Context

Please select one of the following:

• This closes an existing Issue, Closes: Add common extends values to the JSON Schema suggestions #40439
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

AI assistance disclosure

Did you use AI tools to create any part of this pull request?

Please select one option and, if yes, briefly describe how AI was used (e.g., code, tests, docs) and which tool(s) you used.

• No — I did not use AI for this contribution.
• Yes — minimal assistance (e.g., IDE autocomplete, small code completions, grammar fixes).
• Yes — substantive assistance (AI-generated non‑trivial portions of code, tests, or documentation).
• Yes — other (please describe): Claude Sonnet 4.5 (GitHub Copilot + crush)

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests, but ran on a real repository, or
• Both unit tests + ran on a real repository

The public repository:

[jamietanna]

Before:

    "extends": {
      "description": "Configuration presets to use or extend.",
      "oneOf": [
        {
          "type": "array",
          "items": {
            "type": "string"
          }
        },
        {
          "type": "string"
        }
      ]
    },

After:

    "extends": {
      "description": "Configuration presets to use or extend.",
      "oneOf": [
        {
          "type": "array",
          "items": {
            "anyOf": [
              {
                "enum": [
                  "config:best-practices",
                  "config:recommended",
                  "mergeConfidence:all-badges",
                  "abandonments:recommended",
                  "group:all",
                  "replacements:all",
                  "security:minimumReleaseAgeNpm",
                  "security:only-security-updates"
                ]
              },
              {
                "type": "string"
              }
            ]
          }
        },
        {
          "type": "string"
        }
      ]
    },

Tested in Neovim and I can see that:

[github-actions]

🎉 This PR is included in version 43.56.0 🎉

The release is available on:

• GitHub release
• 43.56.0

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