fix(react-refresh): check FunctionDeclaration nodes properly

[aleclarson]

Description

The problem:

export const MyComponent = () => {...}

// This should prevent this module from being a Refresh boundary, but it doesn't.
export function useThing() {...}

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.