Fallback to a default language if unknown

[kmbauer]

Allow the developer to configure a default language to fallback to if not in the supported list.

For example lets assume the markdown

` ` `term
class test{
    System.out.println("Hello world");
}
` ` `

And I configure a fallback language with something like...

.use(shiki, {fallbackLanguage: 'java')

would result in

<pre><code class="language-java">
class test{
    System.out.println("Hello world");
}
</code></pre>