chore: add exports fields

[tolking]

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fix vant-nuxt#31

unimport 中解析 vant 的目录大致为 ../../../node_modules/.pnpm/[email protected][email protected]/node_modules/vant/lib/vant.cjs，这使得自动导入的方法没有类型提示

增加导入后解析的目录变为 ../../../node_modules/.pnpm/[email protected][email protected]/node_modules/vant 类型提示正常