[eslint-config] Remove rig ignore stopgap and eliminate config casts

[Ian Clanton-Thuon (iclanton)]

Summary

Follow-ups to #6006 / #6026, now that @rushstack/eslint-config@4.8.0 is published and pulled back into the repo.

Remove the build-output-ignore stopgap from decoupled-local-node-rig

#6006 temporarily duplicated the build-output ignores (lib, lib-*, dist, temp, coverage) into the rig, because the published @rushstack/eslint-config did not yet ignore them and the rig consumes the published version. 4.8.0 now globally ignores those folders, so the rig-level duplication is removed — consumers inherit the ignores from the base profile.

Eliminate avoidable casts in the eslint-config flat profile

• Replace globalIgnores(...) as Linter.Config with plain { name, ignores } config objects, which are directly assignable to Linter.Config (and remove the now-unused globalIgnores import). These are ordinary flat-config global-ignores objects, so behavior is unchanged.
• Use as object as ESLint.Plugin (rather than as unknown as) in the friendly-locals mixin, for consistency with the profile.

The one remaining @typescript-eslint plugin cast is unavoidable: a direct as ESLint.Plugin fails with TS2352 because typescript-eslint's RuleModule types are intentionally not assignable to ESLint's RuleDefinition. It stays as the tighter as object as ESLint.Plugin.

Not included: TypeScript conversion of the rig's ESLint configs

We considered converting decoupled-local-node-rig's ESLint configs to TypeScript (as was done for @rushstack/eslint-config) but decided against it:

• The rig is a build-tooling package accessed ~54 ways by literal/package paths (tsconfig extends, and heft/jest/api-extractor extends via package specifiers). Standing up a TS build would require either an exhaustive/fragile exports map (risking heft/jest/api-extractor resolution) or an in-place build — a lot of surface area/risk to type-check a handful of tiny config files that are already runtime-validated by every lint run.
• The rig is already a thin overlay that consumes @rushstack/eslint-config and selectively overrides it (its localCommonConfig has 0 rules that merely duplicate the published profile; it is 5 intentional overrides plus 19 repo-specific additions).

How it was tested

• heft build in @rushstack/eslint-config: passed (TypeScript + self-lint)
• heft test --clean in build-tests/eslint-9-test: passed; build output (lib-commonjs/lib-esm) is still ignored and eslint.config.js is still linted (confirms the ignores are inherited from the base profile after removing the rig stopgap)
• rush rebuild for @rushstack/eslint-plugin (a decoupled-rig consumer): self-lints clean
• rush change --verify