Add Playfair Mono

Font Project Git Repo URL:

https://github.com/jhaisley/Playfair-Mono

Super short description of the Font Family:

Playfair Mono is a monospaced variant of Claus Eggers Sørensen's popular transitional serif design, Playfair. While preserving the distinctive high contrast, delicate serifs, and historic aesthetics of late 18th-century pointed steel pen typography, Playfair Mono locks all glyphs to a fixed advance width of exactly 700 units. It features a variable font format interpolating across Weight (Light to Black) and Optical Size (6pt to 1200pt).

Requirements:

By opening this issue, I confirm the project meets the following requirements:

The entire font project is available in a Github repository (repo) and licensed under the [OFL](https://openfontlicense.org/open-font-license-official-text/)
The source files are available in the repo
I am the sole copyright author of the entire project, or all other copyright authors have licensed their work to me under the OFL, and I commit to clearly disclosing if AI tools were used in the creation of this project.
There are no "Reserved Font Names" in the OFL license information, or in the project documentation of any known upstream projects.
The family name is unique according to [namecheck.fontdata.com](https://namecheck.fontdata.com/)
The name of the font family expected to appear on app menus must be very clearly communicated and definitive. It should not include any copyright holder's full names or acronyms.
The font supports at least the Google Fonts 'Latin Core' glyphset from [github.com/googlefonts/glyphsets](https://github.com/googlefonts/glyphsets) ([direct link](https://github.com/googlefonts/glyphsets/blob/main/data/results/txt/nice-names/GF_Latin_Core.txt))
The repo has the [Google Fonts preferred upstream repo structure](https://googlefonts.github.io/gf-guide/upstream.html)
I have read, agree with, and comply with, the full [Google Fonts contributing requirements](https://googlefonts.github.io/gf-guide/index#pre-production-getting-your-fonts-ready-for-gf)
I will maintain the repository and participate in the onboarding process (addressing, solving, and responding to issues, merging pull requests, etc)

Start with the linked Playfair Mono repository and the Google Fonts contributing requirements named in the issue. Verify the OFL licensing, source files, Latin Core glyph support, and preferred upstream structure; done means the font completes the stated onboarding requirements and is ready for the Google Fonts process.

Written by the indexing model from the issue text.