Add option to pass functions as values for TSX HTML element attributes:
```
lang=tsx
const html = <button onclick={ ()=>{ alert("clicked!") } }></button>;
```
The functions should be stringified with `fn.toString()`.
When run on backend, Typescript is going to be complaining about the types within the functions - for example, `alert` does not exist in node and TS is going to throw an error. For now we're going to work around it with `declare` or `// @ts-ignore` and we'll deal with the types in the future.