feat: casbinJsGetPermissionForUser

[Abingcbc]

Fix: #247

[casbin-bot]

@tangyang9464 @seriouszyx @elfisworking please review

[codecov-commenter]

Codecov Report

Merging #248 (28341c1) into master (15d4437) will increase coverage by 0.27%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master     #248      +/-   ##
==========================================
+ Coverage   65.80%   66.08%   +0.27%     
==========================================
  Files          44       45       +1     
  Lines        2214     2229      +15     
  Branches      395      397       +2     
==========================================
+ Hits         1457     1473      +16     
  Misses        640      640              
+ Partials      117      116       -1     

Impacted Files	Coverage Δ
...rc/main/java/org/casbin/jcasbin/main/Frontend.java	93.33% <93.33%> (ø)
src/main/java/org/casbin/jcasbin/util/Util.java	68.18% <0.00%> (+2.27%)	⬆️

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