Update icons for slightly better sizing (#5013)
Changes utils/vscode/images/icon.png to 256x256 (["The path to the icon
of at least 128x128 pixels (256x256 for Retina
screens)."](https://code.visualstudio.com/api/references/extension-manifest#fields)),
and updates the favicon.png to use the same source image.
For reference, changing this to:
https://docs.google.com/drawings/d/16V_E_LS7zqZu6VkNdZgIMqSwc96eV_fw98nIYoY1bBg/edit
The image changes slightly because I don't recall my prior source (and
it didn't seem important to precisely match font size), but hopefully
the drawing helps.
Jon Ross-Perkins
