Simplify newline handling of comment token in TokenStream#to_html

[tompng]

The logic of newline handling to make html <span> tag pretty does not need to be restricted to comment token.
Remove comment-specific condition and make it simple.

What this code is doing

Moves trailing "\n" outside of closing </span> tag to make generated HTML pretty.

<pre>
<span># comment1
</span><span># comment2
<span><span>42</span>
</pre>

↓

<pre>
<span># comment1</span>
<span># comment2</span>
<span>42</span>
</pre>

Background

In most case, only :on_comment token (and weird percent literal string token) might end with "\n" because RDoc::Parser::Ruby has token-squashing.
I'm writing a new syntax highlighter for RDoc and want to make this pretty-html generation applied to other tokens too.
Removing token-squashing will generate more tokens that ends with "\n".

[matzbot]

🚀 Preview deployment available at: https://64804086.rdoc-6cd.pages.dev (commit: a9c8214)