[Basic theme] Icons and svg variations generator

[TSI-nareshrai]

We have replaced nextcloud icons with magenta icons using sass variables only

[tsdicloud]

Please use standard Nextcloud generative approach and CSS recommendation:
https://docs.nextcloud.com/server/latest/developer_manual/html_css_design/css.html#css-variables
https://help.nextcloud.com/t/scss-support-dropped-with-nextcloud-25/141471

This need to reuse Nextcloud icon generator for SASS and the support of the upstream commands:

 "sass": "sass --style compressed --load-path core/css core/css/ apps/*/css",
  "sass:icons": "babel-node core/src/icons.js"
 ``

[tsdicloud]

Please don't separate this PR from #17, pictures and icon generation should be kept together.

[tsdicloud]

Please plan also a separate server PR/branch so that we can upstream our recommended changes on the icon generator.
(summary: differentiate between a replaced custom icon and an newly introduced one which also needs aliases etc.)

[tsdicloud]

Please make sure that the following best practices of Nextcloud are followed:
nextcloud/server#32060