mruby-compiler: allow compound statement in tLPAREN_ARG

Change the grammar rule for tLPAREN_ARG from accepting only a
single stmt to accepting compstmt. This allows compound
statements with semicolons inside parenthesized arguments when
the parenthesis is preceded by a space, e.g., `p (f1; f2)`.

This matches the behavior of CRuby 3.3+.

Fixes #6766.

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

mrbgems/mruby-compiler/core/parse.y

@@ -3260,7 +3260,7 @@ primary         : literal
                       $<stack>$ = p->cmdarg_stack;
                       p->cmdarg_stack = 0;
                     }
-                  stmt {p->lstate = EXPR_ENDARG;} rparen
+                  compstmt {p->lstate = EXPR_ENDARG;} rparen
                     {
                       p->cmdarg_stack = $<stack>2;
                       $$ = $3;