Override server logging level with environment variable (#653)

Use `LOG_LEVEL` environment variable to override logging level for `emu.grasscutter` (which also contains all loggers under it). This might help with debugging various issues reported by users. Previously, the only way to override these levels would be to use `-Dlogback.configurationFile`

src/main/resources/logback.xml

 <Configuration>
+  <variable name="LOG_LEVEL" value="${LOG_LEVEL:-INFO}" />
+
   <appender name="STDOUT" class="emu.grasscutter.utils.JlineLogbackAppender">
     <encoder>
       <pattern>[%d{HH:mm:ss}] [%highlight(%level)] %msg%n</pattern>
@@ -14,11 +16,12 @@
       <pattern>%d{yyyy-MM-dd'T'HH:mm:ss'Z'} - %m%n</pattern>
     </encoder>
   </appender>
-  <logger name="org.reflections" level="OFF"/>
+
+  <logger name="org.reflections" level="OFF" />
+  <logger name="emu.grasscutter" level="${LOG_LEVEL}" />
+
   <root level="INFO">
     <appender-ref ref="STDOUT" />
     <appender-ref ref="FILE" />
   </root>
-  <logger name="emu.grasscutter.scripts.ScriptLib" level="DEBUG">
-  </logger>
 </Configuration>
\ No newline at end of file