7657: utf8 r=matklad a=matklad
- Prepare for utf-8 offsets
- reduce code duplication in tests
- Make utf8 default, implement utf16 in terms of it
- Make it easy to add additional context for offset conversion
- Implement utf8 offsets
closes#7453
Co-authored-by: Aleksey Kladov <aleksey.kladov@gmail.com>