List multiple taxonomy terms with commas in between

... realizing we could probably do this for *any* inline field, not just entity
references.

agaric/templates/field/field--entity-reference.html.twig

@@ -0,0 +1,71 @@
+{#
+/**
+ * @file
+ * Theme override for a field.
+ *
+ * To override output, copy the "field.html.twig" from the templates directory
+ * to your theme's directory and customize it, just like customizing other
+ * Drupal templates such as page.html.twig or node.html.twig.
+ *
+ * Instead of overriding the theming for all fields, you can also just override
+ * theming for a subset of fields using
+ * @link themeable Theme hook suggestions. @endlink For example,
+ * here are some theme hook suggestions that can be used for a field_foo field
+ * on an article node type:
+ * - field--node--field-foo--article.html.twig
+ * - field--node--field-foo.html.twig
+ * - field--node--article.html.twig
+ * - field--field-foo.html.twig
+ * - field--text-with-summary.html.twig
+ * - field.html.twig
+ *
+ * Available variables:
+ * - attributes: HTML attributes for the containing element.
+ * - label_hidden: Whether to show the field label or not.
+ * - title_attributes: HTML attributes for the title.
+ * - label: The label for the field.
+ * - multiple: TRUE if a field can contain multiple items.
+ * - items: List of all the field items. Each item contains:
+ *   - attributes: List of HTML attributes for each item.
+ *   - content: The field item's content.
+ * - entity_type: The entity type to which the field belongs.
+ * - field_name: The name of the field.
+ * - field_type: The type of the field.
+ * - label_display: The display settings for the label.
+ *
+ * @see template_preprocess_field()
+ */
+#}
+{#
+  Ensures that the visually hidden option for field labels works correctly.
+  @todo: Remove when https://www.drupal.org/node/2779919 is resolved.
+#}
+{%
+  set title_classes = [
+    'label',
+    label_display == 'visually_hidden' ? 'visually-hidden',
+  ]
+%}
+{% set title_attributes = title_attributes.addClass(title_classes) %}
+{# Child templates can specify classes to be added to items. #}
+{% set item_classes = item_classes ?? [] %}
+
+{% if label_hidden %}
+  {% for item in items %}
+    {{ item.content}}
+  {% endfor %}
+{% elseif label_display == 'inline' %}
+  <p{{ attributes.removeClass('content') }}>
+    <label{{ title_attributes.removeClass('label') }}>{{ label }}</label>
+    {% for item in items %}
+      <strong>{{ item.content }}</strong>{% if not loop.last %}, {% endif %}
+    {% endfor %}
+  </p>
+{% else %}
+  <p{{ attributes }}>
+    <label{{ title_attributes }}>{{ label }}</label>
+    {% for item in items %}
+      <strong>{{ item.content }}</strong>
+    {% endfor %}
+  </p>
+{% endif %}