Add new import/export system for JavaScript

[rudolf101]

Description

Add support for generating tests for multi-file projects.
Add new tests for multi-file projects.

How to test

Manual tests

Various scenarios have been tested. You can find the examples in the folder `/utbot-js/samples' and try to run UTBot.

Self-check list

• I've set the proper labels for my PR (at least, for category and component).
• PR title and description are clear and intelligible.
• I've added enough comments to my code, particularly in hard-to-understand areas.
• The functionality I've repaired, changed or added is covered with automated tests.
• Manual tests have been provided optionally.
• The documentation for the functionality I've been working on is up-to-date.

[Markoutte]

I cannot get any test for scenarioMultyClassNoTopLevel.js in any method. Also, has an exception:

java.lang.UnsupportedOperationException: No test cases were generated for fDel
	at api.JsTestGenerator.makeTestsForMethod(JsTestGenerator.kt:204)
	at api.JsTestGenerator.run(JsTestGenerator.kt:142)
	at org.utbot.intellij.plugin.language.js.JsDialogProcessor$createTests$1.run(JsDialogProcessor.kt:195)