[examples] Add Dylint integration example

[andyscott]

Summary

• add a self-contained Dylint integration example under examples/dylint
• keep Dylint-specific rules local to the example rather than exposing a core rules_rs API
• demonstrate target-local custom lint selection with two independent Dylint configs

Stacked on #122. Until that PR lands, GitHub will show the prerequisite
rustc-dev toolchain changes in this draft PR as well. The intended review
scope for this PR is only the example layer added on top of #122.

Motivation

The example shows how a downstream repository can integrate Dylint with generated rules_rs toolchains once nightly rustc-dev support is available, without making Dylint itself a first-class ruleset feature.

Details

• add a standalone nested Bazel module at examples/dylint
• keep the Dylint helper rules and runner beside the example as local recipe code
• configure the example to use the nightly rustc-dev toolchain path
• verify that two Rust targets built from the same source can select different Dylint libraries through target-local lint configs
• exclude the nested module from root //... traversal via .bazelignore

Validation

• reviewed as a stacked follow-up on top of the nightly rustc-dev toolchain PR
• root-workspace bazel query //examples/dylint:* confirms the nested module is excluded from the top-level workspace