cli: ability to disable default configs

[dpc]

My motivation is mostly to address #5098 , but in a lot of software there's a flag to disable sourcing default configs, which is useful in a lot contexts.

A more granular (or just atlernative/better) approaches would require more design and debate, while this seems simple and immediately useful (at least to some).

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added tests to cover my changes

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[martinvonz]

Thanks, but JJ_CONFIG= jj should already have this effect

[dpc]

@martinvonz The JJ_CONFIG controls the user's config, no? While I'm trying to disable the default configs shipped with jj.

[martinvonz]

Oh, I'm sorry. Interesting. We have a lot of code that depends on the default configs existing, so I think it's going to be more complicated to support than this PR's current state.

[dpc]

@martinvonz I just noticed. Oops.

As a first user who'd want this, I don't mind just adding all the required keys to my config, and thus being sure I am in full control.

Also (or/and), I could add unwrap_or_default() on most obvious ones.

Also (or/and) I could add JJ_CONFIG_NO_DEFAULT_COLORS.

And (or/and), some config toml could be considered "essential", and loaded even with JJ_NO_DEFAULT_CONFIG.

[dpc]

This seems to have no issues.