nuxt fix(kit): don't use default export of defu

🔗 Linked issue

❓ Type of change

[ ] 📖 Documentation (updates to the documentation or readme)
[x] 🐞 Bug fix (a non-breaking change that fixes an issue)
[ ] 👌 Enhancement (improving an existing functionality like performance)
[ ] ✨ New feature (a non-breaking change that adds functionality)
[ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

For a reason that I don't really understand, I got the following error while using loadNuxt in an external (non-nuxt) vite server:

node-pre-gyp ERR!  TypeError: (0 , import_defu.default) is not a function
node-pre-gyp ERR!     at getOptions (/home/runner/work/JabRefOnline/JabRefOnline/node_modules/@nuxt/kit/dist/index.mjs:136:20)
node-pre-gyp ERR!     at normalizedModule (/home/runner/work/JabRefOnline/JabRefOnline/node_modules/@nuxt/kit/dist/index.mjs:163:28)
node-pre-gyp ERR!     at installModule (/home/runner/work/JabRefOnline/JabRefOnline/node_modules/@nuxt/kit/dist/index.mjs:352:9)
node-pre-gyp ERR!     at async initNuxt (file:///home/runner/work/JabRefOnline/JabRefOnline/node_modules/nuxt/dist/index.mjs:2044:7)
node-pre-gyp ERR!     at async loadNuxt (file:///home/runner/work/JabRefOnline/JabRefOnline/node_modules/nuxt/dist/index.mjs:2077:5)
node-pre-gyp ERR!     at loadNuxt (/home/runner/work/JabRefOnline/JabRefOnline/node_modules/@nuxt/kit/dist/index.mjs:519:19)

For some reason the line in the distributed file, import defu, { defu as defu$1 } from 'defu'; results in these issues. I fixed it locally by using only the name export, and this is what this PR is doing.