(self-contained build) Remove unused css styles

[BenReim]

Expected Behavior

When analyzing the code coverage (e.g. by Chrome Dev Tools --> Coverage) of an UI5 app that has been bundled by the ui5-tooling / self-contained build, a lot of unused styles can be found that add to the size of a build result. Such styles could be omitted by matching the styles with selectors that are actually used in an app, in a similar way how tools like purgecss work.

Current Behavior

All styles will be part of a self-contained build result.

Affected components

• [x] ui5-builder
• [ ] ui5-server
• [ ] ui5-cli
• [ ] ui5-fs
• [ ] ui5-project
• [ ] ui5-logger

[RandomByte]

A little bit related to https://github.com/SAP/ui5-builder/issues/160