Add source of fonts for administration toolbar

Problem/Motivation

Goal of this issue is to standardize font family for administration toolbar.

In toolbar module we have custom fonts declared like Source Sans Pro. However it's not a system font and there is no sources of this font, so on my Ubuntu machine fallback sans-serif font is used instead.

You can see how administration toolbar is looking with Source Sans Pro font by using demo_umami profile installation with enabled Umami theme. Just visit homepage. Here (in Umami) we have sources of "Source Sans Pro" font. And i find it unclear why font in administration toolbar is different in Umami for example when you visiting user page (like homepage) and administrative pages (like on /admin/content page for example).

It can be an issue also with custom themes. So i would recommend maybe to follow "Claro" style font family:
BlinkMacSystemFont, -apple-system, "Segoe UI", Roboto, Oxygen-Sans, Ubuntu, Cantarell, "Helvetica Neue", sans-serif

On the other hand it would be nice to have some custom font with its sources stored directly in Toolbar core module - it will improve user experience across the platforms they using to play with toolbar. If this font can be still "Source Sans Pro" - we can just copy/paste sources from Umami theme.