Asynchronous Functions in Expressions - Introduce an API to determine the execution status of asynchronous functions

[JaneSjs]

T21125 - Asynchronous Expressions and Their Execution Status
https://surveyjs.answerdesk.io/internal/ticket/details/T21125

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Usage scenario: a form uses asynchronous functions for vaiours expression fields such as visibleIf and requiredIf. However, Form Library doesn't currently handle this and, for instance, it is possible to submit a form without waiting until the requiredIf function finishes its execution. Consider the following sample demo: View Demo. It is also possible to call the survey.completeLastPage() without waiting until the async expression function finishes executing: View Demo.

Please consider supporting this usage scenario.

• Block navigation elements if an async function is being executed;
• Add an API which would allow developers to determine the execution status of asynchronous functions. This is required for a custom Save action.