New chunking classes

[dluc]

The original chunkers ported from SK had some bugs introduced while refactoring, leading to incorrect split. This is a full rewrite following the original logic, with some changes:

• remove MaxTokensPerLine setting
• overlap doesn't use sentences anymore, and copy raw tokens from the previous chunk instead
• markdown chunker uses better splitting logic, although it should be rewritten to use a markdown parser
• chunkers now work with a Chunk class which is used also by the file parsers. This will allow to port properties from files to chunks, such as page number and other metadata
• chunkers now take a dependency on tokenizers directly, rather than just TokenCount
• chunkers are now out of Core and into a dedicated nuget, for future reuse outside KM