feat: skip UTF-8 BOM in source input and add regression test

Add handling for UTF-8 byte order mark (BOM) at the start of source code
in wrenCompile function by checking for the three-byte sequence \xEF\xBB\xBF
and advancing the source pointer past it. Include a regression test file
(test/regression/520.wren) that contains a BOM to verify the fix works
correctly.

src/vm/wren_compiler.c

@@ -3420,6 +3420,12 @@ void definition(Compiler* compiler)
 ObjFn* wrenCompile(WrenVM* vm, ObjModule* module, const char* source,
                    bool isExpression, bool printErrors)
 {
+  // Skip potential UTF-8 BOM
+  if (strncmp(source, "\xEF\xBB\xBF", 3) == 0)
+  {
+    source += 3;
+  }
+
   Parser parser;
   parser.vm = vm;
   parser.module = module;
@@ -3447,7 +3453,7 @@ ObjFn* wrenCompile(WrenVM* vm, ObjModule* module, const char* source,
   nextToken(&parser);
 
   int numExistingVariables = module->variables.count;
-  
+
   Compiler compiler;
   initCompiler(&compiler, &parser, NULL, false);
   ignoreNewlines(&compiler);

test/regression/520.wren

@@ -0,0 +1,2 @@
+// This file should have a UTF-8 byte order mark
+System.print("ok") // expect: ok