Add single string usage

[BekYahia]

New Feature

Now you can pass either an array or string via the new prop string

<!-- You can pass an array of strings -->
<vue-writer :string="['Hello World!', 'VueWriter is awesome!']" />
<!-- or a single string -->
<vue-writer string="you can use single string" />

• The old prop array is deprecated and will be removed in the next major version
• This PR has backward compatibility, so array is still usable.

<!-- You can pass an array of strings -->
<vue-writer :array="['Hello World!', 'VueWriter is awesome!']" />
<!-- or a single string -->
<vue-writer array="you can use single string" />

• A Simple demo is added to showcases