Fix rendering of cowsay sheep.cow

Summary:
In the templates of the external cowsay library, most replaceable tokens are specified as $var_name.
However, the sheep.cow and flaming-sheep.cow use the ${eyes} syntax instead. This is not recognised by PhutilCowsay.php resulting in incorrect rendering of the template.

This change updates PhutilCowsay.php to handle ${var_name} tokens as well as $var_name

cowsay(cow='sheep'){{{My eyes, my eyes!}}}

Test Plan:
In a Remarkup comment or document, add
```
cowsay(cow='sheep'){{{How do my eyes look now?}}}
```

When testing in differential, you don't even need to submit the comment.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25435

src/utils/PhutilCowsay.php

@@ -71,14 +71,19 @@ final class PhutilCowsay extends Phobject {
         $template);
     }
 
-    $template = preg_replace_callback(
+    $token_patterns = array(
       '/\\$([a-z]+)/',
+      '/\\${([a-z]+)}/',
+    );
+    foreach ($token_patterns as $token_pattern) {
+      $template = preg_replace_callback(
+        $token_pattern,
         array($this, 'replaceTemplateVariable'),
         $template);
       if ($template === false) {
         throw new Exception(
-        pht(
+          pht('Failed to replace template variables while rendering cow!'));
-          'Failed to replace template variables while rendering cow!'));
+      }
     }
 
     $lines = $this->text;

src/utils/__tests__/cowsay/sheep.expect

@@ -0,0 +1,11 @@
+ __________________
+< How are my eyes? >
+ ------------------
+  \
+   \
+       __
+      UooU\.'@@@@@@`.
+      \__/(@@@@@@@@@@)
+           (@@@@@@@@)
+           `YY~~~~YY'
+            ||    ||

src/utils/__tests__/cowsay/sheep.test

@@ -0,0 +1,13 @@
+  $thoughts
+   $thoughts
+       __
+      U${eyes}U\.'@@@@@@`.
+      \__/(@@@@@@@@@@)
+           (@@@@@@@@)
+           `YY~~~~YY'
+            ||    ||
+~~~~~~~~~~
+{
+  "text": "How are my eyes?",
+  "eyes": "oo"
+}