Add support for nested multi-line comments

For example, this is a nested multi-line comment:

    /*
      Here's a multi-line comment
      /* It explains what this function does */
      It shouldn't appear in any output
    */

If the comment isn’t closed at the end of the file, that’s fine.

src/com/craftinginterpreters/lox/Scanner.java

@@ -80,6 +80,8 @@ class Scanner {
             if (match('/')) {
                 // A comment goes until the end of the line.
                 while (peek() != '\n' && !isAtEnd()) advance();
+            } else if (match('*')) {
+                multiline_comment();
             } else {
                 addToken(SLASH);
             }
@@ -155,6 +157,34 @@ class Scanner {
         addToken(STRING, value);
     }
 
+    private void multiline_comment() {
+        int depth = 0;
+
+        while (!isAtEnd()
+               && (peek() != '*' || peekNext() != '/' || depth != 0)) {
+            if (peek() == '\n') line++;
+
+            if (peek() == '/' && peekNext() == '*') {
+                depth++;
+                advance();
+            }
+
+            if (peek() == '*' && peekNext() == '/') {
+                depth--;
+                advance();
+            }
+
+            advance();
+        }
+
+        if (isAtEnd()) {
+            return;
+        }
+
+        advance();
+        advance();
+    }
+
     private boolean match(char expected) {
         if (isAtEnd()) return false;
         if (source.charAt(current) != expected) return false;