Add type assertion to _kwarg function return

[eliascarv]

On the main branch, this expression does not throw an error:

julia> @truthtable p & (~q | r) full=1
TruthTable
┌───────┬───────┬───────┬───────┬────────┬──────────────┐
│   p   │   q   │   r   │  ¬q   │ ¬q ∨ r │ p ∧ (¬q ∨ r) │
├───────┼───────┼───────┼───────┼────────┼──────────────┤
│ true  │ true  │ true  │ false │ true   │ true         │
│   ⋮   │   ⋮   │   ⋮   │   ⋮   │   ⋮    │      ⋮       │
│ false │ false │ false │ true  │ true   │ false        │
└───────┴───────┴───────┴───────┴────────┴──────────────┘
                                           6 rows omitted

This PR solve this issue by adding a type assertion to _kwarg function return:

julia> @truthtable p & (~q | r) full=1
ERROR: TypeError: non-boolean (Int64) used in boolean context
Stacktrace:
  [1] _kwarg(expr::Expr)
...