Reads a while statement
```ebnf
while ::= T_WHILE (statement | ':' inner_statement_list T_ENDWHILE ';')
```
@see https://github.com/php/php-src/blob/master/Zend/zend_language_parser.y#L587
@return {While}
function() {
    const result = this.node("while");
    this.expect(this.tok.T_WHILE) && this.next();
    let test = null;
    let body = null;
    let shortForm = false;
    if (this.expect("(")) this.next();
    test = this.read_expr();
    if (this.expect(")")) this.next();
    if (this.token === ":") {
      shortForm = true;
      body = this.read_short_form(this.tok.T_ENDWHILE);
    } else {
      body = this.read_statement();
    }
    return result(test, body, shortForm);
  }