Open query text as a .ql file

This fixes a minor bug (introduced in https://github.com/github/vscode-codeql/pull/1115/files#diff-cf43a74569f5e6eea483a9178f5de8fc372f8ca652017d3f1451dd2640876874L603) where we accidentally removed the `.ql` file extension. I've hard-coded it here, and tested it works for the local, remote, and variant-analysis cases.

extensions/ql-vscode/src/query-history.ts

@@ -1073,7 +1073,7 @@ export class QueryHistoryManager extends DisposableObject {
     const queryId = getQueryHistoryItemId(finalSingleItem);
 
     const uri = Uri.parse(
-      `codeql:${queryId}?${params.toString()}`, true
+      `codeql:${queryId}.ql?${params.toString()}`, true
     );
     const doc = await workspace.openTextDocument(uri);
     await window.showTextDocument(doc, { preview: false });