fix: Don't override rules from user's config

[melusc]

If the user disables any of n/no-unsupported-features/* or otherwise overrides it in their config it will now keep that setting.

Fixes #598 and #760
Closes #672

[melusc]

I haven't looked into it, but I would imagine

xo/lib/options-manager.js

Line 364 in 112e657

config.baseConfig.rules['@typescript-eslint/indent'] = ['error', spaces, {SwitchCase: 1}];

and similar lines should also use ??= instead to respect the user's config. I can make a PR or include that in this PR if desired

[sindresorhus]

I can make a PR or include that in this PR if desired

Would be great if you could include it in this PR :)

[melusc]

I have updated them all to use nullish coallescing assignments. I haven't added any more tests. I think if something changes that would undo this the test I already added should catch anything.