fix(cli): always exit with 0 on cdk diff

[jogold]

Add the --fail flag to exit with 1 if there's a diff.

Closes #2111

BREAKING CHANGE: cdk diff now exits with 0 even when there's a diff, use --fail to exit with 1.

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[eladb]

@jogold since this is breaking the stable CLI API, we decided to revert this PR and I would like to propose the following change (let me know if this is acceptable):

1. Make the default of diff --fail to true, so the current behavior will remain the same.
2. When new projects are initialized with cdk init generate their cdk.json file with diff-fail: false by default.

This way, new projects will get the new behavior and existing projects will not experience a break.
Would be nice to implement this cdk.json defaults in a way that it's reused across all init templates.

What do you think?

@RomainMuller @rix0rrr - would love your input here.

[hoegertn]

This sounds great. See my comment on the reverting PR.

[jogold]

2. generate their cdk.json file with diff-fail: false by default.

Is this syntax inside cdk.json already working?

[eladb]

Is this syntax inside cdk.json already working?

Not sure to be honest... I think some custom handling will be needed, but not sure.

[jogold]

I suggest splitting this into 2 different PR's: one for the --no-fail (see #4721) and one for the default for new projects (to follow, needs more work on merging argv with cdk.json).