Improve and enhance default snippets

[subecho]

This patch improves the reliability and readability of the default
Kotlin snippets. Also added a snippet for creating a function with an
arbitrary number of parameters.

[nishtahir]

Thanks for your contribution. I like the changes to the parameter names but I'm not really sure about how the keys are named.

I think it's appropriate to make them a little more verbose, but having them as long as the descriptions seem redundant in my option.

Perhaps we can conform to the same pattern used here? https://github.com/xabikos/vscode-javascript/blob/master/snippets/snippets.json

[subecho]

Oooh, I like what they did with the Javascript snippets. I'll see if I can update the Kotlin ones to conform to the same. Thanks for the feedback!