fix: correct InterleavedBuffer.set offset parameter optionality

[eunjiyun]

Summary

• Aligns InterleavedBuffer.set() offset parameter with the JavaScript implementation.
• Keeps the change limited to the declaration file.

Evidence

• JS implementation: three.js/src/core/InterleavedBuffer.js:191
• Declaration: types/three/src/core/InterleavedBuffer.d.ts:107
• Mismatch: offset has a default value of 0 in JS but is declared as required in the type declaration.

Check

• Checked the declaration diff manually.
• pnpm test failed: unrelated ESLint plugin error.