The ConsoleLine's `cursor` stores the index of the char where the
cursor is currently at. This works for ASCII characters, which
are all one-byte long, but will be offset when the string contains
multi-byte UTF8 sequences.
This caused an issue during auto-complete where the matching results
would be offset to the right.
This commit reuses the same logic as the text editor's
`current_character` RNA property, so that on getting and setting, the
cursor index is converted from and to UTF-8.
Ref: !114121
402cec5646