Reads constant list
```ebnf
constant_list ::= T_CONST (constant_declaration ',')* constant_declaration
```
function(flags) {
    if (this.expect(this.tok.T_CONST)) {
      this.next();
    }
    const result = this.node("classconstant");
    const items = this.read_list(
      /**
       * Reads a constant declaration
       *
       * ```ebnf
       *  constant_declaration ::= (T_STRING | IDENTIFIER) '=' expr
       * ```
       * @return {Constant} [:link:](AST.md#constant)
       */
      function read_constant_declaration() {
        const result = this.node("constant");
        let constName = null;
        let value = null;
        if (
          this.token === this.tok.T_STRING ||
          (this.php7 && this.is("IDENTIFIER"))
        ) {
          constName = this.node("identifier");
          const name = this.text();
          this.next();
          constName = constName(name);
        } else {
          this.expect("IDENTIFIER");
        }
        if (this.expect("=")) {
          value = this.next().read_expr();
        }
        return result(constName, value);
      },
      ","
    );

    return result(null, items, flags);
  }