Update vscode recommendations (#707)

I think the MS C++ extension is what was causing vscode to hang for me. I've been trying clangd and it seems to work better.

yash is syntax highlighting for ypp/lpp. The builtin markdown support's gotten better, so I'm removing the non-standard recommendation.

.vscode/extensions.json

@@ -1,9 +1,9 @@
 {
   "recommendations": [
     "bazelbuild.vscode-bazel",
+    "daohong-emilio.yash",
     "esbenp.prettier-vscode",
-    "ms-python.python",
-    "ms-vscode.cpptools",
-    "shd101wyy.markdown-preview-enhanced"
+    "llvm-vs-code-extensions.vscode-clangd",
+    "ms-python.python"
   ]
 }