[New] extensions: allow enforcement decision overrides based on specifier

[Xunnamius]

This PR implements in import/extensions: a new option enabling developers to override the decision to enforce or ignore extensions for imports with matching specifiers.

A demo package containing this feature is temporarily available for easy testing:

npm install --save-dev eslint-plugin-import@npm:@-xun/eslint-plugin-import-experimental

Prevent false negatives when checking the extensions of bespoke imports

I already use import/order with custom pathGroups. This allows me to control the grouping of certain paths, such as custom aliases from tsconfig.json's paths. This in turn ensures all imports are sorted properly, even when the project is using syntactically sound but otherwise bespoke custom aliases like my-project:api.js (in lieu of ../../../../src/api.js).

Unfortunately, if I mistype my-project:api (i.e. I forgot the extension), the import/extensions rule will still pass; the enforcement decision is "ignore" when it should have been "enforce". This is because, like import/order, the grouping strategy employed by this rule is quite coarse-grained, but unlike import/order, this rule is missing a pathGroups-like setting for finer-grained sorting. The goal of this PR is to fill that gap.

The documentation updates associated with the changes in this PR are part of a separate PR and can be previewed here (towards the bottom of the section). This PR also includes tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.16%. Comparing base (1dc101b) to head (7a69297).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3105      +/-   ##
==========================================
+ Coverage   90.45%   95.16%   +4.71%     
==========================================
  Files          84       84              
  Lines        3634     3643       +9     
  Branches     1279     1285       +6     
==========================================
+ Hits         3287     3467     +180     
+ Misses        347      176     -171     

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[ljharb]

What happens if two contradictory pathGroupOverrides options end up applying to the same file?

[Xunnamius]

What happens if two contradictory pathGroupOverrides options end up applying to the same file?

It mirrors the behavior of pathGroups from import/order: the first path group with a matching minimatch pattern wins.

[Xunnamius]

@ljharb Did this PR need any further work?

[ljharb]

@Xunnamius there's a few lines in the diff that aren't covered by tests.