Add `Frex Sans` (械黑)

**Font Project Git Repo URL:**

https://github.com/maoken-fonts/flex-sans

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

_Flex Sans_ is a typeface based on _IBM Plex CJK_. IBM Plex is an open-source font project released by IBM in 2017. Initially, it was announced that corresponding CJK (Chinese, Japanese, Korean) fonts would be created, but after multiple delays, the Japanese version was not released until 2021. The Traditional Chinese version released in 2024 continued the unique design style of the Latin part. However, the Simplified Chinese version (IBM Plex Sans SC) faced heavy criticism due to numerous unnecessary modifications and design issues. In April 2025, IBM released the source code for IBM Plex fonts. The Flex Sans GB Project Authors took this opportunity to address the issues in IBM Plex Sans SC and created a variable font version with Chinese characters, which is named Flex Sans. More details about this project can be viewed on [maoken.com](https://maoken.com/freefonts/26125.html).

**Requirements:**

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

- [x] The entire font project is available in a Github repository (repo) and licensed under the [OFL](https://openfontlicense.org/open-font-license-official-text/)
- [x] The source files are available in the repo
- [x] ~~I am the sole copyright author of the entire project, or,~~ all other copyright authors have licensed their work to me under the OFL
- [x] There are no "Reserved Font Names" in the OFL license information, or in the project documentation of any known upstream projects. If there are RFNs, they are not used in whole or in part in this family name, or, I want to discuss how Google can work with my use of them.
- [x] The family name is unique according to [namecheck.fontdata.com](https://namecheck.fontdata.com/)
- [x] 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.
- [x] 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))
- [x] The repo has the [Google Fonts preferred upstream repo structure](https://googlefonts.github.io/gf-guide/upstream.html)
- [x] 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)
- [x] ~~I~~ Upstream will maintain the repository and participate in the onboarding process (addressing, solving, and responding to issues, merging pull requests, etc)

**Image:**