[Feature request] Limit the values of `type` in edit_rstudio_snippets() to reasonable options only

[GegznaV]

As I understand, RStudio has a limited number of different types of snippets:

But edit_rstudio_snippets() argument type can have any value. For example, once I forgot that I have to use "markdown" and not "rmarkdown" and created a wrong type of document. Snippets in it did not have an effect.
I expect that only reasonable values of type should be accepted. In other cases, either a warning or an error message should be issued.

[jennybc]

Sure, referring to some list of valid snippet types sounds like a reasonable idea.