Require ECMAScript private class members

[Bharat Middha (bmiddha)]

Summary

• add @rushstack/prefer-ecmascript-private-members
• require native # syntax for private fields, methods, and accessors
• leave private constructors and constructor parameter properties unchanged
• enable the rule in legacy and flat shared ESLint profiles
• document runtime-privacy rationale and unsafe-autofix boundary

Dependency

Draft/stacked on the integration branch for #5939, #5940, #5941, #5942, and #5943. Retarget to main after those migrations merge.

Validation

• rush build --to @rushstack/eslint-config --verbose
• rush test --only @rushstack/eslint-plugin --only @rushstack/eslint-config --verbose
• 208 ESLint plugin tests passed

[Bharat Middha (bmiddha)]

Scope update: the new ESLint rule now enforces ECMAScript #private syntax for private fields, methods, getters, and setters. Private constructors and constructor parameter properties remain excluded because they have no direct native-private equivalent. The refreshed stack passes all 208 ESLint plugin tests.