[Bug report] The externalLinkIcon configuration item is invalid

[zoujingli]

Description

The externalLinkIcon configuration item is invalid

Reproduction

https://stackblitz.com/edit/vuepress-i1rz4c?file=docs%2F.vuepress%2Fconfig.ts

Used Package Manager

npm

System Info

System:
    OS: Windows 11 10.0.22631
    CPU: (16) x64 AMD Ryzen 7 5800H with Radeon Graphics
    Memory: 10.21 GB / 31.41 GB
  Binaries:
    Node: 20.12.2 - C:\Program Files\nodejs\node.EXE
    Yarn: Not Found
    npm: 10.8.0 - C:\Program Files\nodejs\npm.CMD
    pnpm: Not Found
    bun: Not Found
  Utilities:
    Git: 2.45.1.
  Browsers:
    Chrome: Not Found
    Edge: Chromium (123.0.2420.97)
  npmPackages:
    @vuepress/bundler-vite: next => 2.0.0-rc.11
    @vuepress/bundler-webpack: Not Found
    @vuepress/cli:  2.0.0-rc.11
    @vuepress/client:  2.0.0-rc.11
    @vuepress/core:  2.0.0-rc.11
    @vuepress/markdown:  2.0.0-rc.11
    @vuepress/shared:  2.0.0-rc.11
    @vuepress/utils:  2.0.0-rc.11
    vue:  3.4.27
    vue-router:  4.3.2
    vuepress: next => 2.0.0-rc.11