Reads a list of constants declaration
```ebnf
declare_list ::= IDENTIFIER '=' expr (',' IDENTIFIER '=' expr)*
```
@retrurn {Array}
function() {
    const result = [];
    while (this.token != this.EOF && this.token !== ")") {
      this.expect(this.tok.T_STRING);
      const directive = this.node("declaredirective");
      let key = this.node("identifier");
      const name = this.text();
      this.next();
      key = key(name);
      let value = null;
      if (this.expect("=")) {
        value = this.next().read_expr();
      }
      result.push(directive(key, value));
      if (this.token !== ",") break;
      this.next();
    }
    return result;
  }