FEAT #14456 TIME 10 add flat tree material plugin

src/frontend/app/app-common.module.ts

src/frontend/plugins/tree/maarch-flat-tree.component.html

src/frontend/plugins/tree/maarch-flat-tree.component.scss

src/frontend/plugins/tree/maarch-flat-tree.component.ts

src/lang/lang-en.json

src/lang/lang-fr.json

src/lang/lang-nl.json