Adds support for GA4 Google Analytics (alshedivat#727)

* Update `analytics.html`

* Update `config.yml`

* Update

* Update

Author: rohandebsarkar

_config.yml

@@ -91,7 +91,7 @@ contact_note: >
 # Analytics and search engine verification
 # -----------------------------------------------------------------------------
 
-google_analytics:  # your google-analytics ID (format: UA-XXXXXXXXX)
+google_analytics:  # your Goole Analytics measurement ID (format: G-XXXXXXXXXX)
 panelbear_analytics:  # panelbear analytics site ID (format: XXXXXXXXX)
 
 google_site_verification:  # your google-site-verification ID (Google Search Console)

_includes/scripts/analytics.html

@@ -3,12 +3,12 @@
   <script async src="https://www.googletagmanager.com/gtag/js?id={{ site.google_analytics }}"></script>
   <script>
     window.dataLayer = window.dataLayer || [];
-    function gtag() { dataLayer.push(arguments); }
+    function gtag(){ window.dataLayer.push(arguments); }
     gtag('js', new Date());
     gtag('config', '{{ site.google_analytics }}');
   </script>
-  {%- endif -%}
-  {%- if site.enable_panelbear_analytics -%}
+{%- endif -%}
+{%- if site.enable_panelbear_analytics -%}
   <!-- Panelbear Analytics - We respect your privacy -->
   <script async src="https://cdn.panelbear.com/analytics.js?site={{site.panelbear_analytics}}"></script>
   <script>