Add front-end support for case expressions boolean side conditions

[janmasrovira]

• Syntax for #2804.

This pr introduces:

1. front-end support (parsing, printing, typechecking) for boolean side conditions for branches of case expressions.
2. Now if is a reserved keyword.

Example:

 multiCaseBr : Nat :=
    case 1 of
      | zero
        | if 0 < 0 := 3
        | else := 4
      | suc (suc n)
        | if 0 < 0 := 3
        | else := n
      | suc n if 0 < 0 := 3;

The side if branches must satisfy the following.

1. There must be at least one if branch.
2. The else branch is optional. If present, it must be the last.

Future work:

1. Translate side if conditions to Core and extend the exhaustiveness algorithm.
2. Add side if conditions to function clauses.