Transition OnEnter to WorkspaceSnippetEdit

This also changes our handiling of snippet edits on the client side.
`editor.insertSnippet` unfortunately forces indentation, which we
really don't want to have to deal with. So, let's just implement our
manual hacky way of dealing with a simple subset of snippets we
actually use in rust-analyzer

editors/code/src/rust-analyzer-api.ts

@@ -69,7 +69,7 @@ export interface JoinLinesParams {
 export const joinLines = request<JoinLinesParams, SourceChange>("joinLines");
 
 
-export const onEnter = request<lc.TextDocumentPositionParams, Option<SourceChange>>("onEnter");
+export const onEnter = request<lc.TextDocumentPositionParams, Option<lc.WorkspaceEdit>>("onEnter");
 
 export interface RunnablesParams {
     textDocument: lc.TextDocumentIdentifier;