Group common methods into Singleton classes

[zFernand0]

Is your feature request related to a problem? Please describe.

In an effort to reduce the maintenance efforts of Zowe Explorer, it might be convenient to group related methods under a single class. This should help streamline how imports are handled when developing new features and how tests are written for these new methods.

Describe the solution you'd like

Ideally we will use singleton classes 😋

Describe alternatives you've considered

We can consider using:

• Classes with static methods: may have its own drawbacks (not all of which may apply in our case)
• Namespaces: may cause issues when writing jest-tests for method that are not explicitly exported (medium)

Additional context

Suggestion:

• fix: Refactor extension.ts  #2080 (comment)
  • Another conclusion on the same PR was to use named exports instead of default whenever possible 😋

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[zFernand0]

We could address this comment at the same time we handle this issue. 😋