Multiple user configs are now supported and are loaded in the following precedence order:
- `$HOME/.jjconfig.toml`
- `$XDG_CONFIG_HOME/jj/config.toml`
- `$XDG_CONFIG_HOME/jj/conf.d/*.toml`
Which removes the need to set `JJ_CONFIG` for a multi-file approach.
Later files override earlier files and the `JJ_CONFIG` environment
variable can be used to override the default paths.
The `JJ_CONFIG` environment variable can now contain multiple paths separated
by a colon (or semicolon on Windows).
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