ci: add lint workflow

[cakemanny]

It feels a bit naughty to have warnings show on the install, so I was thinking to add a lint check to prevent them for the case of future changes

RUSTFLAGS="-D unused" cargo check

warning: unused import: `ArgAction`
 --> src/main.rs:3:12
  |
3 | use clap::{ArgAction, Parser, Subcommand};
  |            ^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

warning: unused import: `std::str::FromStr`
  --> src/main.rs:12:5
   |
12 | use std::str::FromStr;
   |     ^^^^^^^^^^^^^^^^^

warning: `paseto-cli` (bin "paseto-cli") generated 2 warnings (run `cargo fix --bin "paseto-cli"` to apply 2 suggestions)

I found the way to do it in https://rust-unofficial.github.io/patterns/anti_patterns/deny-warnings.html

It does state that new warnings might pop up with new rust versions and deps, including deprecations so I figured RUSTFLAGS="-D all" cargo check might cause too much friction.
Maybe there are more to added... but I guess clippy has our back for most issues.

I feel it's nicer not to add it to the code because it gets in the way of iteration