An embedded pseudoterminal was enclosed in the EmbedGrid struct. This
commit splits it into EmbedTerminal and EmbedGrid, with EmbedGrid
containing only the CellBuffer grid logic. With this change we can reuse
EmbedGrid to parse ANSI output from external programs into meli's
CellBuffer's.
Merge ui crate with root crate.
In preparation for uploading `meli` as a separate crate on crates.io.
Workspace crates will need to be published as well and having a separate
`ui` crate and binary perhaps doesn't make sense anymore.