refactor: compiler tests

to be more readable and easier to debug
refactor: remove trailing spaces in peg file
feat(syntax) mandatory `<script>` tag for JS content
feat(syntax) enable top-level HTML comments

Now it's possible to have top-level HTML-style comments in the hsp file:

```
<!--
this is
a comment
-->
```

They're emitted at parse time, and discarded at syntax generation time
(replaced with newlines, to keep the 1:1 line mapping between input and
generated file, we may change that in the future).
adds onupdate event handler for input and textarea

Close #310