docs: support dir in meta for pages

[Jinjiang]

Support adding dir in meta like:

---
dir: rtl
---

The demo screenshot

So we can control the direction per each page.

Thanks.

/cc @brc-dd

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[posva]

Shouldn't this be set directly in Vitepress theme 🤔

[Jinjiang]

Yea, I think it's a good idea. Another PR has been created there. :-)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9ac7d5c) 90.85% compared to head (4667994) 90.85%.
Report is 7 commits behind head on main.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2085   +/-   ##
=======================================
  Coverage   90.85%   90.85%           
=======================================
  Files          24       24           
  Lines        1115     1115           
  Branches      347      347           
=======================================
  Hits         1013     1013           
  Misses         63       63           
  Partials       39       39           

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[Jinjiang]

@posva I reset this branch and just bumped the vitepress version. It works as expected from my side. Please take a look. 🙏