Update new encryption scheme & drop support for .NET 6.0

Signed-off-by: Bayu Satiyo <itsyuukunz@gmail.com>

Add new encryption type

This encryption type is come from the Soul Engine
[http://soulengineproject.com/] (is this beta version of Light.vn?)
that I found in HIMAWARI game (https://www.freem.ne.jp/win/game/17154).
Thanks to Zakamutt (https://forums.fuwanovel.net/profile/1088-zakamutt/)
for reporting this.

How does this encryption work?
The `.vndat` file are password protected with the same key as Light.vn
`d6c5fKI3GgBWpZF3Tz6ia3kF0` while the contents of `.vndat` file isn't
encrypted at all. Because of this, I need to rewrite the entire unpack
and repack logic.

Signed-off-by: Bayu Satiyo <itsyuukunz@gmail.com>

Fix xor-ing the last 100 bytes and update README

Using Array.Reverse to automatically reverse KEY and store to
REVERSE_KEY leading to unexpected result to entire decryption process.
When using manually reversed KEY constant (REVERSE_KEY), the problem
is fixed.

Signed-off-by: Bayu Satiyo <itsyuukunz@gmail.com>