Enhance ESLint config

[clemyan]

What's the problem this PR addresses?

Follow up to #6276 and #6352. The ESLint config has room for improvement

• The include patterns are unnecessary wide and causes a lot more files to be linted by default (See chore: lint mjs files and ignore json files #6352, which is only a partial fix)
• The ignore patterns can be simplified (Migrates to Eslint's flat config #6276 (comment))
• There are some extraneous config files and config items
• The @typescript-eslint/naming-convention rule config in @yarnpkg/eslint-config is shadowed by the one in eslint.config.mjs

How did you fix it?

Wide include

The culprit is this line

berry/eslint.config.mjs

Line 43 in 57ed709

files: [`!packages/*/sources/{index,Plugin}.ts`],

By only specifying a negated pattern, it matches ALL other files that are not globally ignored. Under flat config, each file matched by at least one config item (and is not globally ignored) is eligible for linting. This causes the VSCode plugin to attempt to lint pretty much every file.

The correct way to use both files and ignores to accurately specify the set of files to be linted. With that, we can simplify the test:lint command to just lint the whole repo and we have a single source of truth of what should be linted. This also aligns with the default behavior of ESLint v9 for when we migrate. I have also taken the opportunity to widen the typescript files globs to include .cts and .mts if we create those in the future.

Ignore patterns

See #6276 (comment). Added a few missed ignores. Also grouped them by workspace.

Extraneous configs

See #6276 (comment). There are two sets of configs that specify jest globals for tests using env but that is not supported in flat config and has already been migrated in #6276.

@typescript-eslint/naming-convention

The @typescript-eslint/naming-convention is configured twice, once in @yarnpkg/eslint-config and once in eslint.config.mjs. But due to a wide include glob in the latter, the former one is actually almost entirely shadowed -- it only takes effect on sources/index.ts and sources/Plugin.ts of each workspace.

There are already ~100 violations of the rule that went unreported due to this shadowing. I have removed the shadowed config item since it isn't really being enforced. (Or is that actually intentional?)

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.