refactor: create theme-base and fiori packages, and move ShellBar

[vladitasev]

• Create a theme-base package that contains the common css variables for all other packages. This includes component-specific variables that require derived colors. This package uses lessjs for the calculations.
• Create a fiori package, integrate it in the build and move ShellBar.js there.
• The default theme no longer relies on CSS vars fallback values. CSS vars are always explicitly included.
• Update README files to indicate that users may need to install an additional NPM package and all other relevant information.

BREAKING CHANGE: ShellBar.js moved to another package. Please change all references to:
@ui5/webcomponents/dist/ShellBar.js -> @ui5/webcomponents-fiori/dist/ShellBar.js
and
@ui5/webcomponents/dist/ShellBarItem.js -> @ui5/webcomponents-fiori/dist/ShellBarItem.js

[pskelin]

looks good, works locally