Support a global TypeScript / JavaScript switch

We'd like to enable users to choose their preferred language for samples.

The most obvious way to do this would be to let samples include both .ts and .js versions of the same file, and just special case filtering out the other files. This would make it impossible to mix .ts and .js in a single sample though. We could also use another attribute for this, or nest files into another structure.

```html

...
...

```