Support rich object as argument to snippet actions

[pokey]

Something like the following for wrap:

interface NamedSnippet {
  type: "named";
  name: string;
  variableName: string;
}
interface CustomSnippet {
  type: "custom";
  body: string;
  variableName?: string;
  scopeType?: ScopeType;
}
type SnippetDescription = NamedSnippet | CustomSnippet;

async run(
    [targets]: [Target[]],
    snippet: SnippetDescription,
  ): Promise<ActionReturnValue> {}

insert should be similar but no variableName