perf: refactor using semantic HTML (#1207)

Fixes #1196

_includes/datetime.html

@@ -3,12 +3,13 @@
   See: ${JS_ROOT}/utils/locale-dateime.js
 -->
 
-{% assign wrap_elem = include.wrap | default: 'em' %}
 {% assign df_strftime = site.data.locales[include.lang].df.post.strftime | default: '%d/%m/%Y' %}
 {% assign df_dayjs = site.data.locales[include.lang].df.post.dayjs | default: 'DD/MM/YYYY' %}
 
-<{{ wrap_elem }}
-  class="{% if include.class %}{{ include.class }}{% endif %}"
+<time
+  {% if include.class %}
+    class="{{ include.class }}"
+  {% endif %}
   data-ts="{{ include.date | date: '%s' }}"
   data-df="{{ df_dayjs }}"
   {% if include.tooltip %}
@@ -16,4 +17,4 @@
   {% endif %}
 >
   {{ include.date | date: df_strftime }}
-</{{ wrap_elem }}>
+</time>