In the snippet docs there's syntax for adding choices for placeholders:

Choice

Placeholders can have choices as values. The syntax is a comma separated enumeration of values, enclosed with the pipe-character, for example ${1|one,two,three|}. When the snippet is inserted and the placeholder selected, choices will prompt the user to pick one of the values.

Currently the SnippetString class has methods for appendPlaceholder, appendVariable, etc. but there isn't an option for choices. We can build these manually, but it'd be better if we could stick to a nice API.