feat: support key up and down to select variable item

[iamjoel]

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods

[codecov]

Codecov Report

❌ Patch coverage is 91.01124% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.57%. Comparing base (0c8dec3) to head (1b1d7a4).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...s/_base/components/variable/var-reference-vars.tsx	90.47%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #35527      +/-   ##
==========================================
- Coverage   85.57%   85.57%   -0.01%     
==========================================
  Files        4440     4440              
  Lines      207777   207845      +68     
  Branches    38824    38841      +17     
==========================================
+ Hits       177806   177861      +55     
- Misses      26850    26863      +13     
  Partials     3121     3121              

Flag	Coverage Δ
dify-ui	93.01% <ø> (ø)
web	86.50% <91.01%> (-0.01%)	⬇️

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[CodingOnStar]

LGTM