added variables and default values

defaults/main.yml

 ---
-php_module_config_file: "module.ini.j2"
-php_module_priority: <module prio> 
+php_module_config_file: "mbstring.ini.j2"
+php_module_priority: 10
+php_mod_mbstring_language: "neutral"
+php_mod_mbstring_detect_order: "auto"
+php_mod_mbstring_substitute_character: "none"
+php_mod_mbstring_encoding_translation: false
+php_mod_mbstring_http_output_conv_mimetypes: "^(text/|application/xhtml\\+xml)"
+php_mod_mbstring_strict_detection: false

vars/Debian-10.yml

 ---
-package_packages: ["php7.3-<module>"]
+package_packages: ["php7.3-mbstring"]

vars/main.yml

 ---
-php_module_name: "<module name>"
+php_module_name: "mbstring"