Add `str_utf8_offset_bytes_to_chars` and `str_utf8_offset_chars_to_bytes` functions to base system to convert between byte and UTF-8 character offsets in UTF-8 strings.
Previously, this was separately implemented in the textrender and in the lineinput helper.
These textrender functions are entirely replaced by the new functions:
- `ITextRender::SelectionToUTF8OffSets` (by `str_utf8_offset_chars_to_bytes`)
- `ITextRender::UTF8OffToDecodedOff` (by `str_utf8_offset_bytes_to_chars`)
- `ITextRender::DecodedOffToUTF8Off` (by `str_utf8_offset_chars_to_bytes`)
These lineinput helper functions are reimplemented using the new functions:
- `CLineInput::OffsetFromActualToDisplay` (uses `str_utf8_offset_bytes_to_chars`)
- `CLineInput::OffsetFromDisplayToActual` (uses `str_utf8_offset_chars_to_bytes`)
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