Add grammar for execute commands and parser to identify possible next
tokens for the user's execute command input.
The grammar is given as a sequence of Tokens in each command's
definition. The parser parses the user's input according to this
grammar, and returns the tokens that could come next, if any.
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.
Manos Pitsidianakis