Replace more <!> w/ notFollowedBy. Don't use <?>

<?> is N/A because we should never have errors.
Review use of `try`.

I'd done this for blocks, but missed some inlines. Again, the idea is
to localize the try with each parser, since only the parser can know
if it needs to look ahead and backtrack.
Disable $raw-HTML for now.
Add test: Parse random text.

Any input should parse successfully. In other words, any text is a
valid Markdown format file. Try random string and check there is no
parse error exception.