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

crates/rust-analyzer/src/lsp_ext.rs

@@ -102,7 +102,7 @@ pub enum OnEnter {}
 
 impl Request for OnEnter {
     type Params = lsp_types::TextDocumentPositionParams;
-    type Result = Option<SourceChange>;
+    type Result = Option<SnippetWorkspaceEdit>;
     const METHOD: &'static str = "rust-analyzer/onEnter";
 }