feat(lint): add ignoreNull option to noDoubleEquals rule

[peaBerberian]

Note: this is my first contribution/proposal to biome, and I was unsure of if I had to open an issue/discussion first for a relatively simple proposal like this one, or if writing it directly to not waste time was OK. Do not hesitate to tell me the preferred etiquette for potential future contributions, thanks.

Summary

The noDoubleEquals rule made an exception when checking against null, as it's a frequently used pattern to check against both null and undefined.

This not something that is always wanted by developers, some preferring no exception to this rule in the name of explicitness.
I also often seen == null used as a shortcut when people were unsure if some nullable value could also be undefined due to other factors not catched by a type checker (out-of-bounds array indexing or Record lookup on inexistent properties for example), in which case I do also prefer making such shortcuts less tempting to allow simpler future code updates.

The original eslint rule, eqeqeq, has an always option to disable specific handling of null. Here, I propose an ignoreNull option instead, by default set to true to not break API (and as it may be your recommended setting), because I found it much more explicit on what it does than the more vague always, though I don't really have any attachment to that naming if you prefer another one.

For implementation and documentation, I tried to look at how rules like useNamingConvention declared options and try to rely on more or less the same syntax and macros for its struct.

Test Plan

I added tests for invalid cases under the ignoreNull: false case, that I named invalidNoNull in the corresponding directory. I checked that their output seemed right.

[codspeed-hq]

CodSpeed Performance Report

Merging #3702 will not alter performance

_{Comparing peaBerberian:feat/suspicious-no-double-equals-authorize-null-option (a63ee2d) with main (58d1cdc)}

Summary

✅ 101 untouched benchmarks

[arendjr]

This is a great contribution, thank you!

As for the etiquette, filing an issue in advance, isn't required, but it's recommended to avoid filing PRs the maintainers may disagree with.

[arendjr]

Thanks!

[shuhaowu]

Thanks for this feature. It helps a lot when trying to migrate to biome. One thing is there are also some project that standardizes on using == undefined instead of == null to check for undefined or null. Is it possible to allow that case as well?

[Conaclos]

Thanks for this feature. It helps a lot when trying to migrate to biome. One thing is there are also some project that standardizes on using == undefined instead of == null to check for undefined or null. Is it possible to allow that case as well?

Note that by default Biome already ignores == and != against null. ignoreNull was introduced for opting in ESLint always mode.
ESLint doesn't allow == and != against undefined in any mode.
However, by symmetry with null, I am not opposed to also allow == and != against undefined by default.
If we make the change, we should certainly replace ignoreNull with a more general option (maybe strict?).

We could also add a new rule that ensure using consistently ==/!= with null or undefined.