Add themer.dev to Programming Tools

[mjswensen]

https://themer.dev

Themer is a login-less progressive web app that allows users to generate matching themes and desktop wallpaper for many popular programming tools. It should be included in this list because users do not have to create an account to use the software. It is also installable and offline-capable.

By submitting this pull request I confirm I've read and complied with the below requirements.

Failure to properly do so will just result in the pull request being closed and everyone's time wasted. Please read it twice. Most people miss many things.

• [x] I have read the Contribution guidelines and I am confident that my PR is suitable.
• [x] This pull request has a descriptive title. For example, Add [App name], not Update readme.md or Added new entries.
• [x] The app I added is not a duplicate.