Add front-end support for case expressions boolean side conditions

[janmasrovira]

• Syntax for Side conditions in pattern matching #2804.
• ⚠️ Depends on Merge if -> ite renaming from stdlib #2869.

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.
3. Multiway if is allowed to have only the else branch. I've also refactored the parser to be simpler.

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.

[lukaszcz]

Can we have a test with multiple if conditions for one pattern? I understand this is allowed.

[janmasrovira]

done