Preserve newlines after blocks of comments.

This is an example of output from the formatter now:

  ```
  //// Some module documentation

  // foo
  const a: Int = 42

  // Some comment block
  // For which newlines are respected.
  // Foo

  // Another one

  /// add_one documentation
  pub fn add_one(n: Int) -> Int {
    // n + 1
    n + 1
  }
  ```

  before this commit, comments would all be collapsed into one group
  above the function as:

  ```
  // Some comment block
  // For which newlines are respected.
  // Foo
  // Another one
  /// add_one documentation
  pub fn add_one(n: Int) -> Int {
  ```

crates/lang/src/parser.rs

@@ -47,7 +47,6 @@ pub fn module(
             false
         }
         Token::EmptyLine => {
-            println!("{:?}", span);
             extra.empty_lines.push(span.start);
 
             false

crates/lang/src/parser/lexer.rs

@@ -117,10 +117,17 @@ pub fn lexer() -> impl Parser<char, Vec<(Token, Span)>, Error = ParseError> {
             .map_with_span(|token, span| (token, span)),
     );
 
+    let comments_with_trailing_newline = just("//").ignore_then(
+        take_until(text::newline())
+            .then_ignore(text::newline().rewind())
+            .to(Token::Comment)
+            .map_with_span(|token, span| (token, span)),
+    );
+
     choice((
         module_comments,
         doc_comments,
-        comments,
+        choice((comments_with_trailing_newline, comments)),
         choice((keyword, int, op, grouping, string))
             .or(any().map(Token::Error).validate(|t, span, emit| {
                 emit(ParseError::expected_input_found(