core: `diff` should imply `--fail` by default

### Describe the bug

Rather than a separate `--fail` flag, `cdk diff` should return a non-zero exit code if there is a diff. This would provide more information to the end user, making it easier to automate `cdk diff` commands.

### Expected Behavior

`cdk diff` should return zero if there is no difference, and a non-zero exit code if there is a difference.

### Current Behavior

`cdk diff` returns a zero exit code if there is a difference.

### Reproduction Steps

Run `cdk diff` on a stack with differences.

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### CDK CLI Version

2.93.0 (build 724bd01)

### Framework Version

_No response_

### Node.js Version

v18.17.1

### OS

NixOS

### Language

Typescript

### Language Version

5.2.2

### Other information

_No response_

The issue names the cdk diff CLI entry point but no source file or test. First run cdk diff on a stack with differences and on one without, then trace the command's exit-status handling. Done means no differences return zero and differences return non-zero.

Written by the indexing model from the issue text.