[selesse]

Fixes #299. This picks up #373 from @oehlschl (their commit is kept as-is) and addresses the review there, so it should also let us close #300. Loosely related to #463, but they don't depend on each other.

Right now the cache key is the file's mode and contents, the erb_lint config, and the erb_lint version. Editing .rubocop.yml or anything it inherits from, bumping RuboCop or a plugin, or editing .better-html.yml keeps serving stale results until you clear the cache. I like #373's approach, where each linter can return a cache_key and the runner folds those into the cache key. On top of it, this does 4 things:

1. Renames checksum to cache_key and set_runner_checksum to runner_cache_key=, like @etiennebarrie suggested, and adds the specs he asked for, covering each step from a linter's key to a cache miss.
2. Keys the RuboCop linter on RuboCop::ResultCache.source_checksum instead of Gemfile.lock. Hashing the lockfile has the downside @oehlschl pointed out on Break cache if Gemfile.lock or .rubocop.yml change. #300: any dependency bump busts every entry. It's also read from the working directory, so it gets skipped entirely with BUNDLE_GEMFILE or outside Bundler. source_checksum is how RuboCop invalidates its own result cache: it hashes the contents of every loaded file plus the RuboCop and rubocop-ast versions. That covers plugins, cop gems pulled in through require: that aren't named rubocop-* (like shopify-cops), and project-local cops. It only looks at contents, so it's the same across checkouts, and it takes 10–20ms for roughly 2,000 files.
3. Keeps hashing the merged RuboCop config, since it already resolves inherit_from and inherit_gem. RuboCop expands every Exclude in it to an absolute path, though, so the key was different in every checkout. I relativize those against the project root so checkouts sharing a cache directory, like git worktrees, still get hits.
4. Makes ErbSafety key on the contents of its better_html_config file, since editing it (e.g. to mark a helper as safe) had the same staleness problem. Let me know if you'd rather I split this out and keep this PR about RuboCop.

The tradeoff is that any change to a loaded gem's code now invalidates the cache, which is what RuboCop's own cache does too. source_checksum is a public method, but ResultCache is marked @api private, and it only computes anything since RuboCop 1.82.1 (rubocop/rubocop#14730). On older versions it stays nil, so the key falls back to the versions of the loaded rubocop* gems. This branch's lockfile is still on RuboCop 1.70, so the spec that edits a file loaded through require: is skipped here until #465 lands.

On the note in #373 about busting every cache on release: ERBLint::VERSION is already part of the key, so every release does that anyway. I left the runner's key as-is.