Add Function Help Snippet

[JustinGrote]

## PR Summary

This snippet is an improved version of the comment-help snippet using modern features like placeholders and providing more information about what each section means.

I am not replacing comment-help to preserve backwards compatibility for those who are used to it.

Input and Output were also removed because Input is generally specified by the parameters and Output should be specified with the `[OutputType]` attribute, if desired.

## PR Checklist

Note: Tick the boxes below that apply to this pull request by putting an `x` between the square brackets.
Please mark anything not applicable to this PR `NA`.

- [x] PR has a meaningful title
- [x] Summarized changes
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[PrzemyslawKlys]

I would love if there was a functionality that allows to auto-add properties after the initial insert of comment based-help using ###. Do you think this is somehow feasible?

[andyleejordan]

I could actually use some help cleaning up our snippets...e.g. #3404 and #3276.

[JustinGrote]

@PrzemyslawKlys Yes, I want something similar to Document This but for PowerShell, where you type <### after a function name and it will auto-fill as much as it can. I think it's feasible to build it as a separate extension rather than part of this one, but that's not in the scope of this.
https://marketplace.visualstudio.com/items?itemName=oouo-diogo-perdigao.docthis

@andschwa what is your tolerance on "breaking changes" in the snippets? Should everything always be net-new, or should we save the old ones off and provide instructions in the README how to import the "classics" into their user snippets if they want them?

[andyleejordan]

Tolerance is high! Do it. This is the time and place especially with the big Preview happening.

[JustinGrote]

Would be neat if there was a way for a PS module to contribute its own snippets via a path in the .psd1 privatedata file too, that's for another time though.

[JustinGrote]

Closing in favor of a broader snippet refactor per discussion