Syntax for incorporating variables in command argument

Goal is to enable injecting in the commands values coming from

- some user input (see #404)
- cargo, eg the MSRV to have a `cargo +1.77 check` task (see #407)
- variables defined in the bacon.toml file (may be overriden per task, prefs, etc.)
- bacon itself (eg the name of the last failing test)
- other...

I'm not settled on a syntax right now but it must support arguments.

Maybe something like

```TOML
[jobs.ex-ask]
vars.MSRV = "cargo::msrv"
command = [
"cargo",
"+${MSRV}",
"check",
"--all-targets",
]
```

```TOML
# A task dedicated to checking cargo can be compiled with the MSRV
[jobs.check-msrv]
vars.example_name = {
source = "user::input",
question = "Name of the example to run?"
}
command = ["cargo", "run", "--example", "${example_name}"]
```

Start by reading the examples in this issue and the related discussions in #404 and #407. Define a variable syntax that supports command arguments and the listed sources, including user input, Cargo values, bacon.toml variables, and bacon-provided values; the work is done when those cases have an agreed, testable design.

Written by the indexing model from the issue text.