fix: allow newlines before then in if/then/else expressions

The parser now skips newlines between the condition and `then` keyword,
enabling multiline if expressions like:
  if long_condition
    then expr1
    else expr2

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/parser.rs

@@ -1792,6 +1792,7 @@ impl Parser {
 
         let condition = Box::new(self.parse_expr()?);
 
+        self.skip_newlines();
         self.expect(TokenKind::Then)?;
         self.skip_newlines();
         let then_branch = Box::new(self.parse_expr()?);