Add missing italics files for Inter

[ryelle]

The global-fonts code has italics for EB Garamond and IBM Plex Mono, but somehow Inter's italics are missing. Browsers will faux-italicize content so it's easy to miss, but there are odd edge cases like https://github.com/WordPress/wporg-developer-blog/issues/15.

Let's add Inter's italics files.

[renintw]

If https://github.com/WordPress/wporg-developer-blog/issues/15#issuecomment-1385568142 holds up, since I saw that EB Garamond and IBM Plex Mono are added to the fonts/, perhaps it helps if we also add the Inter that we used to subset to the folder.