Feature/php8 support (#629)

Adds support for PHP8.0, dropping support for PHP7.0 and below.

Backwards incompatible changes:
- Dropped support for php asp tags in templates (removed from php since php7.0)
- Dropped deprecated API calls that where only accessible through SmartyBC
- Dropped support for {php} and {include_php} tags and embedded PHP in templates. Embedded PHP will now be passed through as is.
- Removed all PHP_VERSION_ID and compare_version checks and conditional code blocks that are now no longer required
- Dropped deprecated SMARTY_RESOURCE_CHAR_SET and SMARTY_RESOURCE_DATE_FORMAT constants
- Dropped deprecated Smarty::muteExpectedErrors and Smarty::unmuteExpectedErrors API methods
- Dropped deprecated $smarty->getVariable() method. Use $smarty->getTemplateVars() instead.
- $smarty->registerResource() no longer accepts an array of callback functions

See the changelog for more details.

Switched CI from Travis to Github CI.

libs/plugins/modifiercompiler.default.php

@@ -11,7 +11,7 @@
  * Name:     default
  * Purpose:  designate default value for empty variables
  *
- * @link   http://www.smarty.net/manual/en/language.modifier.default.php default (Smarty online manual)
+ * @link   https://www.smarty.net/manual/en/language.modifier.default.php default (Smarty online manual)
  * @author Uwe Tews
  *
  * @param array $params parameters
@@ -26,7 +26,7 @@ function smarty_modifiercompiler_default($params)
     }
     array_shift($params);
     foreach ($params as $param) {
-        $output = '(($tmp = @' . $output . ')===null||$tmp===\'\' ? ' . $param . ' : $tmp)';
+        $output = '(($tmp = ' . $output . ' ?? null)===null||$tmp===\'\' ? ' . $param . ' ?? null : $tmp)';
     }
     return $output;
 }