feat: support slash variable filtering in prompt editor

[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 74.46809% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.51%. Comparing base (da00de6) to head (493d521).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
.../components/variable/var-reference-vars.helpers.ts	62.06%	11 Missing ⚠️
...s/_base/components/variable/var-reference-vars.tsx	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #35460      +/-   ##
==========================================
+ Coverage   85.50%   85.51%   +0.01%     
==========================================
  Files        4434     4437       +3     
  Lines      207009   207143     +134     
  Branches    38538    38604      +66     
==========================================
+ Hits       176996   177133     +137     
+ Misses      26904    26901       -3     
  Partials     3109     3109              

Flag	Coverage Δ
dify-ui	93.01% <ø> (+0.15%)	⬆️
web	86.36% <74.46%> (+0.02%)	⬆️

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[CodingOnStar]

LGTM