support specifying `outDir` in `vitepress preview`

Describe the bug

The vitepress build script recognizes the --outDir argument. For example,

• default script: "docs:build": "vitepress build" builds the ./.vitepress/dist folder;
• custom script: "docs:build": "vitepress build --outDir './dist"' builds the ./dist folder.

However, the vitepress preview script cannot locally preview the project in a different folder. For example,

• default script: "docs:preview": "vitepress preview" considers the ./.vitepress/dist folder;
• custom script: there is no way to preview the project that was built on the ./dist folder.

Workaround: setting '../dist' as the value of outDir option on config.js satisfies the expected result when running vitepress preview. The considered folder becomes ./dist as expected.

Reproduction

https://stackblitz.com/edit/vite-tyd3c7?file=package.json

Expected behavior

The script docs:preview correctly recognizes the modified outDir folder.

[brc-dd]

vitepress preview doesn't accept flags via CLI (except for base and port as the docs mention). You need to specify such things in config directly, or you can use some other server like npx serve ./path/to/your/dist which would work too.

vitepress serve is alias of vitepress preview. serve was there earlier, I'll update the command in stackblitz to align more for new users.