Add Common Serif

**Font Project Git Repo URL:**

https://github.com/StefanPeev/Common-Serif

**Super short description of the Font Family:**

Fork of Libertinus, requested in #12 ; would be good to add these together.

**Requirements:**

It looks to me like @stefanpeev has got most of the GF requirements (discussed in https://github.com/StefanPeev/Common-Serif/issues/49 - and he asks for help using the template in https://github.com/StefanPeev/Common-Serif/issues/50)

- [x] I think its work collaborating with Stefan on the [contributing requirements](https://googlefonts.github.io/gf-guide/index#pre-production-getting-your-fonts-ready-for-gf)
- [x] The entire font project is now licensed under the [OFL](https://openfontlicense.org/open-font-license-official-text/) whereas Libertinus has a GPL custom build system (afaik)
- [x] There are "Reserved Font Names" in the upstream Libertinus, currently not mentioned in the OFL license, so I filed https://github.com/StefanPeev/Common-Serif/issues/60 to recommend adding them.
- [ ] Family name is sadly **not unique** according to [namecheck.fontdata.com](https://namecheck.fontdata.com/), and collides with https://fontsinuse.com/typefaces/219567/abc-common-serif - yet this is not listed on https://abcdinamo.com and fontsinuse says it is "to be released" there. I hope @weiweihuanghuang can comment on the name collision and how to proceed.
- [ ] I didn't check if the font supports at least [GF Latin Core glyphset](https://github.com/googlefonts/glyphsets/blob/main/data/results/txt/nice-names/GF_Latin_Core.txt) but I assume so since Libertinus is pretty big.
- [ ] I expect we can collab on the repository [repo structure](https://googlefonts.github.io/gf-guide/upstream.html)
- [x] The source files are available in the repository
- [x] The project is OFL
- [x] The project is actively maintained