Remove sass-build dependency

[Juveniel]

In this PR:

• Removed sass-build dependency along with related config files.
• Removed unused gulp tasks embed-assets and create-component.
• Renamed core/_index.import.scss to core/index.scss as we no longer need to cater to node-sass specific partials.
• Fixed a issue with the compilation of the core package.

Things to do:

• Fix Fluent theme compilation warnings from postcss-calc

[zhpenkov]

This is not part of this PR, but we should evaluate if we need the sass:standalone build, as it compiles all components' index files, which do not compile to any component styles. Currently, the sass: standalone only tests whether the dependency files in the index are imported correctly. The sass script already covers that.

Edited
The sass:standalone script has been removed.

[zhpenkov]

LGTM