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/sysplugins/smarty_resource.php

@@ -72,9 +72,7 @@ abstract class Smarty_Resource
         }
         // try registered resource
         if (isset($smarty->registered_resources[ $type ])) {
-            return $smarty->_cache[ 'resource_handlers' ][ $type ] =
-                $smarty->registered_resources[ $type ] instanceof Smarty_Resource ?
-                    $smarty->registered_resources[ $type ] : new Smarty_Internal_Resource_Registered();
+            return $smarty->_cache[ 'resource_handlers' ][ $type ] = $smarty->registered_resources[ $type ];
         }
         // try sysplugins dir
         if (isset(self::$sysplugins[ $type ])) {