jerry: does not support String.prototype.startsWith()

[PixMod]

ES6又提供了三种新方法。

includes()：返回布尔值，表示是否找到了参数字符串。

startsWith()：返回布尔值，表示参数字符串是否在源字符串的头部。

endsWith()：返回布尔值，表示参数字符串是否在源字符串的尾部。

请支持。

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[yorkie]

@morrowind Please describe your requirements in English, and answer the questions in our templates.

[PixMod]

@morrowind Please describe your requirements in English, and answer the questions in our templates.

我查到ES6官网上的说明，就直接c&p了，下次会用英语。

[yorkie]

And you should answer the questions in our templates:

Is your feature request related to a problem? Please describe. Describe the solution you'd like Describe alternatives you've considered

[yorkie]

https://github.com/jerryscript-project/jerryscript/commit/d4ce20d0fdc09edd30a2d990b743459e84c4801a has been implemented these functions as you want, does anybody wanna land this on ShadowNode?