feat: dropdown to watch for changes in default option prop

[sohonisaurabh]

Description

Dropdown component does not watch for changes in defaultOption prop, when this is changed programatically. This results in display of outdated selected option on UI.
This PR fixes this issue by adding a watcher for changes in defaultOption prop.

Related Issue

Motivation and Context

Discovered this issue while implementing user preferences pane in UX refresh

How Has This Been Tested?

Tested using built bundle in consumer code. And also via unit tests

Screenshots:

Before (If applicable):

After:

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #1317 (671df63) into master (9692f80) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1317      +/-   ##
==========================================
- Coverage   94.10%   94.09%   -0.01%     
==========================================
  Files         132      132              
  Lines       17677    17684       +7     
  Branches     2647     2647              
==========================================
+ Hits        16635    16640       +5     
- Misses       1042     1044       +2     

Impacted Files	Coverage Δ
web-components/src/components/dropdown/Dropdown.ts	88.84% <100.00%> (+0.15%)	⬆️
web-components/src/components/icon/Icon.ts	78.57% <0.00%> (-4.17%)	⬇️
web-components/src/mixins/RovingTabIndexMixin.ts	93.63% <0.00%> (-3.64%)	⬇️
...components/src/components/datepicker/DatePicker.ts	87.45% <0.00%> (-2.67%)	⬇️
web-components/src/components/input/Input.ts	98.87% <0.00%> (-0.38%)	⬇️
...nts/src/components/table-advanced/TableAdvanced.ts	88.10% <0.00%> (+0.46%)	⬆️
web-components/src/components/radio/Radio.ts	97.84% <0.00%> (+2.15%)	⬆️
web-components/src/components/tabs/Tab.ts	100.00% <0.00%> (+5.39%)	⬆️

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