use textContent and innerHTML where appropriate

[Rich-Harris]

Ref #23. Any time you have an element that contains no if/each blocks, tags, components, or elements with bindings/refs/transitions/dynamic attributes, we can just use innerHTML (or textContent, if there's only a single text node child) instead of constructing the DOM programmatically.

There's some evidence that this is slightly slower on microbenchmarks, but I think we're talking about very modest differences. In return, we get leaner, more readable generated code.

We could generate all DOM that doesn't contain if/each blocks this way, like this...

div.innerHTML = "<button>click me</button>";
button = div.children[0];

...but this PR doesn't go that far.

Thoughts?