feat: MultiSearcher and FacetSearcher

[VladislavFitz]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	yes
Need Doc update	yes

Describe your change

• MultiSearcher implementation
• FacetSearcher implementation
• Highlighting extension for Facet
• Official Dart client integration
• Merge helper-dart and helper-flutter into a single library
• Insights becomes a Flutter package

[VladislavFitz]

@aallam I've applied most of your suggestions and added examples. Can you run through the code again?