fix(config): validate custom rules, accept language names, add a dry run - #691

…run (#691)

## What

- `Config::validate` (so config load and `cora config validate`) now
rejects an invalid regex, an empty or duplicate id, an id shadowing a
built-in rule, an empty message, an empty `languages` list and unknown
`languages` values, naming the rule.
- `languages` accepts language names (`rust`) as well as extensions
(`rs`); `languages` and `exclude` are optional.
- `cora config validate --dry-run [--path DIR]` runs each custom rule
over the project and prints how many lines it matched (0 matches is
flagged).
- `docs/configuration.md` shows the real schema (`rules_engine.custom`,
the free-text `rules:` list, required fields); the 0.19.0 changelog line
says `custom`.

## Why

A mistyped pattern or language made a rule silently inactive while
`config validate` said valid. Closes #684, closes #678.

## Testing

- Unit tests for validation, language normalisation and optional YAML
fields.
- 4 end-to-end tests in tests/custom_rules.rs (invalid regex fails
validation, docs example loads and fires, unknown language, dry run
flags a silent rule).
- fmt, clippy -D warnings and the full suite pass.

Signed-off-by: ajianaz <ajianaz@users.noreply.github.com>
Co-authored-by: ajianaz <ajianaz@users.noreply.github.com>