Fix #123: Add support for 'or' operator in case/when tags (#127)

This commit implements support for the 'or' operator in 'when' clauses
of case statements, matching the official Shopify Liquid specification.

Changes:
- Modified expressions.y grammar to accept 'or' as an alternative
  separator to commas in the expr2 rule for when statements
- Regenerated y.go parser from the updated grammar
- Added comprehensive test cases for 'or' operator usage:
  * Integer values with 'or' operator
  * String values matching Shopify documentation examples
  * Multiple 'or' operators (e.g., 1 or 2 or 3)

The implementation now supports both syntaxes as specified in the
Shopify documentation:
- Comma-separated: {% when 1, 2 %}
- OR operator: {% when 1 or 2 %}
- Multiple ORs: {% when 1 or 2 or 3 %}

All existing tests continue to pass, confirming backward compatibility.

Fixes #123

Co-authored-by: Claude <noreply@anthropic.com>

Author: osteele

expressions/expressions.y

@@ -84,6 +84,7 @@ exprs: expr expr2 { $$ = append([]Expression{&expression{$1}}, $2...) } ;
 expr2:
   /* empty */    { $$ = []Expression{} }
 | ',' expr expr2 { $$ = append([]Expression{&expression{$2}}, $3...) }
+| OR expr expr2  { $$ = append([]Expression{&expression{$2}}, $3...) }
 ;
 
 string: LITERAL {

expressions/statements_test.go

@@ -42,4 +42,8 @@ func TestParseStatement(t *testing.T) {
 	stmt, err = ParseStatement(WhenStatementSelector, "a, b")
 	require.NoError(t, err)
 	require.Len(t, stmt.Exprs, 2)
+
+	stmt, err = ParseStatement(WhenStatementSelector, "a or b")
+	require.NoError(t, err)
+	require.Len(t, stmt.Exprs, 2)
 }