Add error_lines and error_style = tree settings

[KaiSforza]

With the new inner error make options, the .show_related_errors_as_nested() setting makes a lot of sense, I but did not want to enable it by default. This adds a new error mode that uses the nested error style. I called it 'tree', but if it makes more sense to call it 'nested' or 'nest' then that's fine as well.

Also added a setting to change the number of context lines shown for the labels. Sometimes this can let labels get squashed into one block of code, making it easier to read.

This also sets a #[default] for the ErrorStyle.

Release notes summary - What our users need to know

• New option for $env.config.error_style to show related/nested errors: tree
• New setting to change the number of lines printed for error context (default: 1): $env.config.error_lines

Tasks after submitting

• Update the documentation

[github-actions]

Hey, just a bot checking in! You edited files related to the configuration.
If you changed any of the default values or added a new config option, don't forget to update the doc_config.nu which documents the options for our users including the defaults provided by the Rust implementation.
If you didn't make a change here, you can just ignore me.

[WindSoilder]

Hi, sorry for waiting, would you please resolve conflicts first?

[WindSoilder]

Thanks!