Add a warning when adding a new parent feature flag

[samsymons]

Task: https://app.asana.com/1/137249556945/project/1199333091098016/task/1212670452887355?focus=true

This PR adds a new check that warns when adding a new feature flag that uses a parent feature flag. The issue it warns about is that parent feature flags don't support rollouts, and if you want one then you should use a subfeature flag.

See duckduckgo/apple-browsers#3064 for an example of this working.

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Note

Adds a Danger check to flag additions of parent feature flags and wires it into PR validation, with unit tests.

• Introduces remoteReleasableFeatureWarning in org/allPRs.ts to scan Swift diffs for added lines containing .remoteReleasable(.feature and issue a warn with guidance
• Integrates the new check into the default Danger run
• Adds tests in tests/remoteReleasableFeatureWarning.allPRs.test.ts covering no Swift changes, no diff, deletions, and additions

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[alessandroboron]

LGTM! Thanks!