Reads a list of constants declaration
```ebnf
const_list ::= T_CONST T_STRING '=' expr (',' T_STRING '=' expr)* ';'
```
function() {
    return this.read_list(
      function() {
        this.expect(this.tok.T_STRING);
        const result = this.node("constant");
        let constName = this.node("identifier");
        const name = this.text();
        this.next();
        constName = constName(name);
        if (this.expect("=")) {
          return result(constName, this.next().read_expr());
        } else {
          // fallback
          return result(constName, null);
        }
      },
      ",",
      false
    );
  }