Add create_if_missing option to add-lines configurator

[OskarStark]

Why

In symfony/ai we basically have an "empty" ai.yaml file, as no stores are configured as default. When adding your first store you are not able to leverage the recipe otherwise.

cc @chr-hertel

Summary

This PR adds a create_if_missing option to the add-lines configurator. When using position: after_target, if the target is not found in the file, the content specified in create_if_missing will be appended to the file, followed by the regular content.

Example usage:

{
    "add-lines": [{
        "file": "config/packages/ai.yaml",
        "position": "after_target",
        "target": "    store:",
        "create_if_missing": "ai:\n    store:",
        "content": "        azuresearch:\n            default:\n                endpoint: '%env(AZURE_SEARCH_ENDPOINT)%'"
    }]
}

Behavior:

• If store: exists → Content is added after it (existing behavior)
• If store: doesn't exist but create_if_missing is set → The create_if_missing content is appended to the file, followed by the content
• If store: doesn't exist and create_if_missing is not set → Warning is logged (existing behavior)

[fabpot]

We can maybe close this PR?

[stof]

My proposal in symfony/recipes#1503 (comment) would make this PR unnecessary (adding one more advantage to the list)