Treat dict log_config without a version key as an overlay on uvicorn defaults.

A partial log_config (no "version" key) is deep-merged over uvicorn's
default config before our patching, so users pass only their
customizations, e.g. log_config={"loggers": {"myapp": {"level":
"DEBUG"}}}.  Previously such dicts crashed at startup: dictConfig
requires a version, and our root entry referenced a "default" handler
that might not exist; that reference is now only added when the handler
exists.  The stock default formatter is replaced only when untouched,
so an overlaid fmt survives.  Dicts with version and non-dict configs
behave as before.  Documented in the README logging section, with tests
under fastapi-vue/tests (each patched config validated through
dictConfig).

fastapi-vue/tests/__init__.py

@@ -0,0 +1 @@
+"""Tests for the fastapi_vue package."""