fix: sass: moves scroll bars mixin to global scope and adds it to dropdown

[dkilgore-eightfold]

SUMMARY:

Moves scroll-bars mixin to global scope, add it to Dropdown module to achieve improved scroll bar styles.

scrollBarsMixin.mp4

Scrollbar should be consistent in webkit browsers. Firefox is the outlier, where I'm still investigating a fix.

JIRA TASK (Eightfold Employees Only):

ENG-68019

CHANGE TYPE:

• Bugfix Pull Request
• Feature Pull Request

TEST COVERAGE:

• Tests for this change already exist (storybook CSS only)
• I have added unittests for this change

TEST PLAN:

Pull the PR branch and run yarn and yarn storybook. Verify the Dropdown, Menu, Select, DatePicker choose time, and TimePicker stories behave as expected.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4b6af71) 85.04% compared to head (52ff0b0) 85.05%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #726   +/-   ##
=======================================
  Coverage   85.04%   85.05%           
=======================================
  Files         999      999           
  Lines       19255    19255           
  Branches     7112     7112           
=======================================
+ Hits        16376    16377    +1     
+ Misses       2797     2796    -1     
  Partials       82       82           

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