[new feature] DropdownMenu: add direction prop

[chenjiahan]

No description provided.

[codecov-io]

Codecov Report

Merging #3490 into dev will increase coverage by 0.32%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             dev    #3490      +/-   ##
=========================================
+ Coverage   92.2%   92.53%   +0.32%     
=========================================
  Files        117      117              
  Lines       2578     2584       +6     
  Branches     223      223              
=========================================
+ Hits        2377     2391      +14     
+ Misses       177      169       -8     
  Partials      24       24

Impacted Files	Coverage Δ
packages/dropdown-item/index.js	89.65% <100%> (+13.65%)	⬆️
packages/dropdown-menu/index.js	100% <100%> (+17.64%)	⬆️
packages/mixins/click-outside.js	83.33% <0%> (+33.33%)	⬆️

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