Add tests for comments in more expression-like contexts

[nex3]

See sass/dart-sass#2263

[skip dart-sass]

[ntkme]

I wonder if we should omit the whitespace in between silent comment and sibling tokens during output if possible?

[nex3]

It's not generally safe to omit that whitespace entirely—for example, foo//\nbar is very different than foobar. There are some cases where we could fold more aggressively than we do now, but that's generally true with newlines adjacent to trailing spaces, not just silent comments.

[Goodwine]

If there are multiple lines with //, are they considered a single silent comment, or multiple silent comments?

For example, would this block:

a {
  b: progid:c(d // foo
                // bar
    );
}

result in this?:

a {
  b: progid:c(d

    );
}

And if so, is that OK?

[nex3]

They are not considered a single comment, but the whitespace around them is still considered "adjacent" so you won't end up with multiple newlines.

[ntkme]

There are some cases where we could fold more aggressively than we do now

If we treat silent comment as a kind of whitespace when processing AST, I think this can potentially allow us to fold it more aggressively based on context, even under the current logic.

For example, if silent comment is treated as whitespace:
a { b: calc(c<whitespace><silent-comment><newline><whitespace>) } can be parsed as a { b: calc(c<whitespace>) }, and this whitespace can be completely striped based on its context.

Not sure if there is any corner case, and I haven't looked at the implementation details so not sure how much work it is. If it's too much work we can just leave it as is.