fix(stepfunctions): allow multiline jsonata strings (#35985)

### Issue #35912

Closes #35912.

### Reason for this change

The inability to use multiline JSONata expressions in certain places, for example:

```
Condition.jsonata(`{%
  false
%}`)
```

### Description of changes

- Add the "dotAll" flag to the private jsonata helper testing whether a string is wrapped in the Step Functions JSONata expression tags. The "s" flag allows the dot to match across newlines, enabling the use of well-formatted complex expressions.
  - Also considered `String.startsWith` and `String.endsWith`, but opted for the minimal change to preserve the existing regex which is also likely more performant.
- Include unit tests for both helper functions in `jsonata.ts`.
- Reuse `isValidJsonataExpression` where the same regex expression was used in `condition.ts` and `task-utils.ts`.


### Describe any new or updated permissions being added

N/A

### Description of how you validated changes

- Unit tests were added including the multiline cases.
- Existing integration test for Step Functions with JSONata was updated to have its Condition use a multi-line string.
- A construct overriding Condition to use this regex for validation was successfully used in a local project to get around the current single-line limitation.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: MathieuGilbert