Adopt SnippetString API

With the December release VS Code will have real API to define a CompletionItem that is inserted as snippet, for details see https://github.com/Microsoft/vscode/issues/3210.

In short, we now support a subset of the more common TextMate syntax: $1, $2 etc define tabstops, ${1:value} and ${2:this is${3:nested}} placeholders, and $0 is for the final tabstop.

The API for this is contained in a new type SnippetString. It can be used as the insertText of a CompletionItem.

During a transition phase we will still support the internal, unspecified snippet syntax your extension is using but eventually remove it (roughly 2 or 3 milestones from now).

[DanTup]

Great, thanks for the heads up!

Note-to-me: Here’s an example of this migration (TypeScript) as a reference.

[DanTup]

I think the issue is that the internal API used underscore as the final cursor position (see https://github.com/Microsoft/vscode/issues/6874#issuecomment-235606255) but in 1.8 it renders a real underscore.