feat: honor /* exported */ in no-unused-vars, prefer-const and no-useless-assignment

[swwind]

Summary

Consumes the inline /* exported */ API added in #1760 in the four remaining rules ESLint applies the directive to: no-unused-vars (core and @typescript-eslint), prefer-const, and no-useless-assignment.

RuleContext.IsExportedGlobalBinding(decl, name) performs the lookup ESLint does before marking a variable: the name is listed by a comment, the file's top level is the global scope, and decl binds the name in that scope. A var reaches the global scope from inside any block, while let/const/class, a function declaration, and a catch parameter belong to their nearest block scope.

Each rule then reads the flag the way its upstream counterpart does:

• no-unused-vars, core and TypeScript, counts the directive as a use. reportUsedIgnorePattern still sees the binding as used, which is what turns a directive on an ignored name into a usedIgnoredVar report. The TypeScript scope manager puts type-only declarations in the same global scope as value ones, so the directive also reaches interface, type, enum, namespace, and import x = require().
• prefer-const declines to judge such a global, since a separately loaded file may reassign it. Under destructuring: "all" that holds the whole destructuring group back.
• no-useless-assignment treats it like an export: the value can still be read from outside the file, so no assignment to it is reported.

Six /* exported */ skips are removed from the core no-unused-vars upstream suite and one from no-useless-assignment's.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[elecmonkey]

The shared scope-gating helper and all four rule integrations match ESLint's exported-directive semantics across global, block, function, module, CommonJS, destructuring, and TypeScript-only declaration shapes. No actionable correctness, regression, performance, or maintainability issue was found.