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.

tests/UnitTests/ConfigFileTests/file/ConfigVarTest.php

@@ -20,7 +20,7 @@ class ConfigVarTest extends PHPUnit_Smarty
      * This method is called before a test is executed.
      *
      */
-    public function setUp()
+    public function setUp(): void
     {
         $this->setUpSmarty(dirname(__FILE__));
     }
@@ -403,11 +403,7 @@ class ConfigVarTest extends PHPUnit_Smarty
             $this->assertEquals("", $this->smarty->fetch('foo.tpl'));
         }
         catch (Exception $e) {
-	        if (PHP_VERSION_ID >= 80000) {
-		        $this->assertStringStartsWith('Undefined variable', $e->getMessage());
-	        } else {
-		        $this->assertStringStartsWith('Undefined variable', $e->getMessage());
-	        }
+            $this->assertStringStartsWith('Undefined variable', $e->getMessage());
         }
     }
 }