✨ Feature: 新增 dotenv 嵌套配置项支持 (#1324)

Co-authored-by: hemengyang <hmy0119@hotmail.com>

tests/.env.test

@@ -1,5 +1,13 @@
 LOG_LEVEL=TRACE
 NICKNAME=["test"]
 SUPERUSERS=["test", "fake:faketest"]
+COMMON_OVERRIDE=new
 CONFIG_FROM_ENV=
 CONFIG_OVERRIDE=old
+NESTED_DICT={"a": 1}
+NESTED_DICT__B=2
+NESTED_DICT__C__D=3
+NESTED_MISSING_DICT__A=1
+NESTED_MISSING_DICT__B__C=2
+NOT_NESTED=some string
+NOT_NESTED__A=1