[0.85] Feature Flag to enable MutationObserver by default (disabled)

[rubennorte]

Target Branch

0.85

Link to commit or PR to be picked

facebook/react-native#56600

Description

We want to test this API internally at Meta in an Expo-based app but we're not ready for a general release. This just adds the feature flag but it's still disabled. Should be a no-op generally.

[github-actions]

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