Config extends should it merge the linter entry

Like we merge emitter options(Override per emitter) I think linter config entry should merge with the value from the config above.

```tsp
// In tspconfig.base.yaml
linter:
extends:
mylinter/all
```

```tsp
// In tspconfig.yaml
extends: tspconfig.base.yaml
linter:
disable:
mylinter/foo: "Exempted"
```

Here it would still pull the same extends as in the base but include the disabled.
This would just merge one level deeper that it does today for linter
- Right now: if you respecify `linter:` it just override the whole entry(Example above drop the extends)
- Porpose: Merge the content of linter. So specifying linter.disable just override linter.disable(We don't merge each disable entry

Locate the configuration inheritance and merge handling, then find the existing tests for emitter-option or linter configuration merging. Reproduce the base and extending tspconfig.yaml example, verify that linter.extends is retained while linter.disable is overridden as a whole, and add coverage for the expected merged result.

Written by the indexing model from the issue text.