Add code snippets

We should provide the user with snippets/boilerplate code to make writing in `r-check` faster.

Examples include:

* A boilerplate for `agent`
* Snippets for `forall/exists` in LTOL 
* Possibly, snippets that activate on `!` that provide boilerplate for send-guard, data, update (might be as simple as `()()[]`)
* The same for `?` but obviously without send guard