Add button in sidebar looks out of place

[miguelsolorio]

Testing microsoft/vscode#69254

The sidebar icons are using Octicons while the add icon is VS Code's plus button, which has a thicker stroke and is larger, which makes it look out of place with the rest of the icons.

Suggestion would be to use the Octicon plus:

[RMacfarlane]

I prefer matching the VS Code plus button to be consistent with the rest of VS Code