Add feature `abishekvashok-cmatrix`

Files:

src/abishekvashok-cmatrix/devcontainer-feature.json
src/abishekvashok-cmatrix/install.sh (make sure to set executable bit)
test/abishekvashok-cmatrix/test.sh
.github/workflows/test.yaml

Releases:

https://github.com/abishekvashok/cmatrix/releases