Add mini-css to the package.json

[Pailowmenah]

Description

Fix build issue with simple-keyboard.

[webpack-cli] Failed to load '/home/phi/repositories/simple-keyboard/webpack.config.js' config
[webpack-cli] Error: Cannot find module 'mini-css-extract-plugin'
Require stack:

npm install mini-css-extract-plugin seemed to resolve this problem .

Testing

• Released a test version of the simple-keyboard and verified core-ui can be built properly.