Error out for non-escaped special chars in `@pattern`s (#2752)

## Motivation and Context
Closes https://github.com/awslabs/smithy-rs/issues/2508

## Testing
Modify `codegen-core/common-test-models/pokemon.smithy`:
```diff
diff --git a/codegen-core/common-test-models/pokemon.smithy b/codegen-core/common-test-models/pokemon.smithy
index 014ee61c4..2dd37046e 100644
--- a/codegen-core/common-test-models/pokemon.smithy
+++ b/codegen-core/common-test-models/pokemon.smithy
@@ -59,9 +59,12 @@ structure GetStorageInput {
     passcode: String,
 }
 
+@pattern("[.\\n\r]+")
+string Species
+
 /// A list of Pokémon species.
 list SpeciesCollection {
-    member: String
+    member: Species
 }
 
 /// Contents of the Pokémon storage.

```

Try to codegen example service and see the error:
```bash
$ cd examples
$ make codegen
...
[ERROR] com.aws.example#Species: Non-escaped special characters used inside `@pattern`.
You must escape them: `@pattern("[.\\n\\r]+")`.
See https://github.com/awslabs/smithy-rs/issues/2508 for more details. | PatternTraitEscapedSpecialChars /smithy-rs/codegen-server-test/../codegen-core/common-test-models/pokemon.smithy:62:1
```
(For some reason validation errors reported by plugins get formatted by
[LineValidationEventFormatter](https://github.com/awslabs/smithy/blob/aca7df7daf31a0e71aebfeb2e72aee06ff707568/smithy-model/src/main/java/software/amazon/smithy/model/validation/LineValidationEventFormatter.java)
but errors reported by smithy-cli formatted by
[PrettyAnsiValidationFormatter](https://github.com/awslabs/smithy/blob/aca7df7daf31a0e71aebfeb2e72aee06ff707568/smithy-cli/src/main/java/software/amazon/smithy/cli/commands/PrettyAnsiValidationFormatter.java).
Might be a bug in smithy-cli)

Replace pattern with `@pattern("[.\\n\\r]+")` and observe that error is
gone:
```bash
$ make codegen
```

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._