Enable package:pedantic lints (#43)

Fix one case of `prefer_is_not_empty`.
diff --git a/analysis_options.yaml b/analysis_options.yaml
new file mode 100644
index 0000000..108d105
--- /dev/null
+++ b/analysis_options.yaml
@@ -0,0 +1 @@
+include: package:pedantic/analysis_options.yaml
diff --git a/lib/src/span_mixin.dart b/lib/src/span_mixin.dart
index 9bf0937..75ecb3d 100644
--- a/lib/src/span_mixin.dart
+++ b/lib/src/span_mixin.dart
@@ -51,7 +51,7 @@
     buffer.write(': $message');
 
     var highlight = this.highlight(color: color);
-    if (!highlight.isEmpty) {
+    if (highlight.isNotEmpty) {
       buffer.writeln();
       buffer.write(highlight);
     }