Description of the new feature/enhancement

There are disparate UTF-8 parsers. One implemented in Utf8ToWideCharParser and one in UTF8OutputPipeReader. The latter makes it rather difficult to unify UTF-8 parsing because it combines reading from a pipe with the handling of partial code units.

Proposed technical implementation details (optional)

Get rid of UTF8OutputPipeReader, move the pipe reading back to ConptyConnection::_OutputThread().

Implement UTF-8 <--> UTF-16 conversion in user mode. Enable to toggle between ignoring invalid UTF-8 and replacing it with U+FFFD. See #3378

Implement a re-usable partials handling.