feat(cli): read environ variable for verbosity (#63)

* feat(cli): read environ variable for verbosity

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

manim_slides/commons.py

@@ -56,5 +56,7 @@ def verbosity_option(function: Callable) -> Callable:
         help="Verbosity of CLI output",
         default=None,
         expose_value=False,
+        envvar="MANIM_SLIDES_VERBOSITY",
+        show_envvar=True,
         callback=callback,
     )(function)