fix: correct UniformGroup.dispose declaration

[eunjiyun]

Summary

• Aligns UniformsGroup.dispose() with the JavaScript implementation.
• Keeps the change limited to the declaration file.

Evidence

• JS implementation: three.js/src/core/UniformsGroup.js:130
• Declaration: types/three/src/core/UniformsGroup.d.ts:34
• Mismatch: the declaration exposed a chainable return value that the implementation does not return.

Check

• Confirmed dispose() returns undefined at runtime.
• pnpm test failed due to an unrelated ESLint plugin error.