refactor: update the ExtensionService APIs

[wewoor]

Description

Refactor the ExtensionService and add unit tests for it.

Changes

• Add getExtension and getAllExtensions methods
• Private the extensions property, only allow access by public methods
• Add remove method
• Add public reset method for set the extensions to empty
• Add comments for IExtensionService interface

How Has This Been Tested?

• Add unit tests for the ExtensionService

[codecov]

Codecov Report

Merging #342 (de84035) into main (20d6b93) will increase coverage by 3.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #342      +/-   ##
==========================================
+ Coverage   52.95%   56.05%   +3.09%     
==========================================
  Files         173      173              
  Lines        4604     4626      +22     
  Branches     1016     1017       +1     
==========================================
+ Hits         2438     2593     +155     
+ Misses       2138     2007     -131     
+ Partials       28       26       -2     

Impacted Files	Coverage Δ
src/services/extensionService.ts	100.00% <100.00%> (+15.15%)	⬆️
src/model/workbench/editor.ts	62.06% <0.00%> (ø)
src/common/dom.ts	66.10% <0.00%> (+3.38%)	⬆️
src/components/select/option.tsx	100.00% <0.00%> (+75.00%)	⬆️
src/components/select/select.tsx	100.00% <0.00%> (+87.69%)	⬆️
src/services/problemsService.ts	100.00% <0.00%> (+92.64%)	⬆️

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[ProfBramble]

+1