[6.x] Fix model MorphToMany relationships not being augmented. (#416)

* Fixed augmentation for MorphToMany relationships.

* Fix style issue

* Style fix attempt 2

* Fixed accidentally removing if statement.

* Don't need these parentheses

---------

Co-authored-by: Duncan McClean <duncan@duncanmcclean.com>

Author: mefenlon

src/Fieldtypes/BaseFieldtype.php

@@ -6,6 +6,7 @@
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
 use Illuminate\Database\Eloquent\Relations\HasMany;
+use Illuminate\Database\Eloquent\Relations\MorphToMany;
 use Illuminate\Support\Arr;
 use Illuminate\Support\Collection;
 use Statamic\CP\Column;
@@ -174,7 +175,7 @@ public function augment($values)
     {
         $resource = Runway::findResource($this->config('resource'));
 
-        if ($values instanceof HasMany) {
+        if ($values instanceof HasMany || $values instanceof MorphToMany) {
             $results = $values
                 ->get()
                 ->map->toAugmentedArray()
@@ -242,7 +243,7 @@ public function shallowAugment($values)
     {
         $resource = Runway::findResource($this->config('resource'));
 
-        if ($values instanceof HasMany) {
+        if ($values instanceof HasMany || $values instanceof MorphToMany) {
             $results = $values
                 ->get()
                 ->map->toShallowAugmentedArray()