Evaluate & remove unnecessary module mocks

[traeok]

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

In a recent PR, I noticed we were adding to a version of the ImperativeError class that's existed in a module mock for quite some time. I don't think this is good practice because classes such as error classes shouldn't need stubbed out, if anything we can use the real implementation and mock out what we need.

Describe the solution you'd like

Look for unnecessary module mocks and remove them to make unit test maintenance easier.
Consider dropping module mocks from vscode.ts in favor of jest-mock-vscode.

Describe alternatives you've considered

We can continue to stub out parts of Imperative and Zowe classes in module mocks, but this introduces separation.

[github-actions]

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