Add a decoder-like type for splitting incrementally-received text

[jwodder]

Prerequisite: #9

• The type should work like a tokio_util::codec::Decoder type where Item is String and Error is std::string::FromUtf8Error.

• A decoder is constructed from a NewlinePattern value and searches a given buffer of bytes for a match; if it finds one, it attempts to decode all bytes up to/through the match using String::from_utf8(), returning the error if it fails.

• Add options for whether to retain trailing matches and/or yield a final empty string after a final match