Future: declaration of time, state, control and variable in the abstract syntax

The idea of this issue is to add possibilities of declaration of time, state, control and variable.

For instance, this current declaration

```julia
@def begin
tf ∈ R, variable
t ∈ [0, tf], time
x = (q, v) ∈ R², state
u ∈ R, control
tf ≥ 0
v(t) ≤ 1
-1 ≤ u(t) ≤ 1
end
```

must be equivalent to

```julia
@def begin
tf ∈ R₊, variable
t ∈ [0, tf], time
x = (q, v) ∈ (-∞, 1] × R, state
u ∈ [-1, 1], control
end
```

In the comments below, you will find:

- [Preliminaries](https://github.com/control-toolbox/CTParser.jl/issues/30): how the code works.
- [Current possibilities](https://github.com/control-toolbox/CTParser.jl/issues/30): how to declare for the moment.
- [Todo](https://github.com/control-toolbox/CTParser.jl/issues/30): the new possibilities.