chore(prettier): ignore lib/vscode (#5347)

We were using an overrides command in our `.prettierrc.yaml` which
quickly became out of sync with Code's Prettier styles.

Instead, we simply tell Prettier to ignore `lib/vscode`.

This way, if you have `formatOnSave` on and you save inside
`lib/vscode`, you won't convert the file to use code-server's styles.

.prettierignore

@@ -0,0 +1 @@
+lib/vscode