feat: replace kong with cobra/viper

`treefmt.toml` has been extended to include an entry for most of the flags that can be passed to `treefmt`.

In addition, values can now be specified via the environment, prefixed with `TREEFMT_`.

Finally, the order of precedence for value lookup has been fixed: flag -> env -> treefmt.toml.

Closes #351

Author: brianmcgee

Diff for: LICENSE

@@ -1,21 +0,0 @@
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