build: add Bazel rules

[CaerusKaru]

• Add Bazel build rules for all entrypoints
• Add Bazel rules to serve the demo app
• Add goldens api guard
• Move from SCSS to CSS for most demo app components
  due to conflict with CLI and Bazel style generation
• Move testing utilities into core (though not exported with the bundle)
• Move other utilities into proper packages
• Create intermediary BaseFlexFlowDirective for use by flex and flex-offset
  and remove unneeded method from BaseDirective2

[CaerusKaru]

Technically blocked on getting SSR tests to work with Bazel, but not actually blocked since we have regular SSR tests that run fine at the moment.

[devversion]

LGTM

[CaerusKaru]

Closing this since it would likely need another rewrite to use Bazel v3.0 anyway.

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