File code insertion

[t-montes]

The component should be able to also render the code inside of a code file, it could be called in both ways.

The current, with inline code:

<code-viewer title="main.js">
console.log("Hi!");
</code-viewer>

Also with a code file:

<code-viewer file="main.js"/>

where main.js contains:

console.log("Hi!");

If the user mistakenly uses both, one should be preferred.