Fixing snippets in new LSP completion

We need to set insertTextFormat in the LSP completion items to Snippet when appropriate otherwise snippets get treated as plain text, resulting in things like

<h1 style='$1'

getting inserted

Author: alexgav

src/razor/src/completion/completionHandler.ts

@@ -10,6 +10,7 @@ import {
     CompletionParams,
     CompletionTriggerKind,
     InsertReplaceEdit,
+    InsertTextFormat,
     InsertTextMode,
     MarkupContent,
     Position,
@@ -344,6 +345,7 @@ export class CompletionHandler {
                 documentation: CompletionHandler.toMarkupContent(completionItem.documentation),
                 filterText: completionItem.filterText,
                 insertText: CompletionHandler.toLspInsertText(completionItem.insertText),
+                insertTextFormat: CompletionHandler.toLspInsertTextFormat(completionItem.insertText),
                 insertTextMode: CompletionHandler.toInsertTextMode(completionItem.keepWhitespace),
                 kind: completionItem.kind ? completionItem.kind + 1 : completionItem.kind, // VSCode and LSP are off by one
                 label: CompletionHandler.toLspCompletionItemLabel(completionItem.label),
@@ -390,6 +392,19 @@ export class CompletionHandler {
         return snippetString?.value ?? <string | undefined>insertText;
     }
 
+    private static toLspInsertTextFormat(insertText?: string | vscode.SnippetString): InsertTextFormat {
+        return insertText instanceof vscode.SnippetString ? InsertTextFormat.Snippet : InsertTextFormat.PlainText;
+    }
+
+    private static toInsertTextMode(keepWhitespace?: boolean): InsertTextMode | undefined {
+        if (keepWhitespace === undefined) {
+            return undefined;
+        }
+
+        const insertTextMode: InsertTextMode = keepWhitespace ? InsertTextMode.asIs : InsertTextMode.adjustIndentation;
+        return insertTextMode;
+    }
+
     private static toLspTextEdit(
         newText?: string,
         range?: vscode.Range | { inserting: vscode.Range; replacing: vscode.Range }
@@ -448,13 +463,4 @@ export class CompletionHandler {
 
         return lspPosition;
     }
-
-    private static toInsertTextMode(keepWhitespace?: boolean): InsertTextMode | undefined {
-        if (keepWhitespace === undefined) {
-            return undefined;
-        }
-
-        const insertTextMode: InsertTextMode = keepWhitespace ? InsertTextMode.asIs : InsertTextMode.adjustIndentation;
-        return insertTextMode;
-    }
 }