Support dividing statement lists into clauses/stanzas

[jaybosamiya-ms]

This PR adds support for dividing statement lists into clauses/stanzas. This prevents the need for the empty // workaround to divide code into stanzas.

Specifically, with this PR, the following code is (now) considered well-formatted:

verus! {

fn fff() {
    reveal(f1);

    foo;

    bar;
    baz;
}

} // verus!

Prior to this PR, verusfmt would have collapsed the additional newlines around foo;. With this PR, we maintain the additional newlines (however if there are too many newlines, we bring it down to just the two, as a stylistic choice).

Note: this only adds support for stanzas for statement lists. It explicitly does not work in general contexts (such as dividing a single expression up via extra newlines). I believe that supporting stanzas at the statement level should cover most people's use cases for them, while limiting the scope of the shenanigans necessary for the parser to be able to handle the special case of the double/multi-newline.

Resolves #43