Expose the private buildPrefix as a replacement for imperative's implementation of `moment.format(...)`

[zFernand0]

The idea is that if an application developer is using Imperative (e.g. Zowe Explorer) they are able to use this function to retrieve the date formatting directly from the Imperative framework instead of having to reinvent the wheel :yum:

Originally posted by @zFernand0 in https://github.com/zowe/imperative/pull/783#pullrequestreview-939946900