Fix tree-sitter grammar to support optional trailing semicolon after …

…else block

Commit 183278c exposed the following
statement to the parser with a trailing semicolon:

            if (val > 50) {
                return green;
            } else if val > 40 {
                return yellow;
            } else {
                return blue;
            };

editors/tree-sitter-slint/grammar.js

@@ -95,7 +95,7 @@ module.exports = grammar({
         choice(
           seq($.assignment_block, optional(";")),
           seq($.assignment_expr, optional(";")),
-          $.if_expr,
+          seq($.if_expr, optional(";")),
           $.callback_event,
           $.binding,
           seq($.expression, optional(";")),