Add feature `icholy-ttygif`

[sebst]

Files:

src/(FeatureName)/devcontainer-feature.json
src/(FeatureName)/NOTES.md
src/(FeatureName)/install.sh (make sure to set executable bit)
test/(FeatureName)/test.sh
.github/workflows/test.yaml
README.md

Releases:

https://github.com/icholy/ttygif/releases

Test the installation process extensively before submitting
Watch for files in releases, naming structure, etc.
Test CLI output extensively before writing the test file.

For NOTES.md:
Link to the project that is installed by the feature
Short description of what it does
Installation method
Other notes

Update README.md (in repo root). Add the feature, keep the alphabetical order