refactor: extract shared rollup configuration to reduce duplication

[Copilot]

Addresses review feedback on #4251 regarding duplicated rollup configuration between stable and next builds.

Changes

• Extract shared external and watch constants
• Add getPlugins(additionalPlugins) helper for plugin configuration
• Add getOutput(cjsFile) helper for output configuration

Before/After

// Before: ~97 lines with full duplication
export default [
  { input: ['./src/index.ts'], external: [...], watch: {...}, plugins: [...], output: [...] },
  { input: ['./src/index.next.ts'], external: [...], watch: {...}, plugins: [...], output: [...] },
]

// After: ~74 lines with shared config
export default [
  { input: ['./src/index.ts'], external, watch, plugins: getPlugins([del(...)]), output: getOutput('./dist/eds-core-react.cjs') },
  { input: ['./src/index.next.ts'], external, watch, plugins: getPlugins(), output: getOutput('./dist/index.next.cjs') },
]

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