pipeline: should work with pnpm

### Describe the feature

Currently the pipeline respects npm and yarn. However, it should also by default respect the pnpm package manager.

### Use Case

pnpm significantly improves our developer experiences. However cdkPipelines doesn't support it.

### Proposed Solution

Update the default CdkSynth to detect `pnpm-lock.yaml` the same way it currently detects `yarn.lock`.

### Other Information

_No response_

### Acknowledgements

- [X] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

2.81.0

### Environment details (OS name and version, etc.)

irrelevant

Start at the default CdkSynth implementation and compare how it detects npm and yarn lockfiles, especially yarn.lock. Update the detection to account for pnpm-lock.yaml, then run the relevant CDK Pipelines tests if available; done means pnpm projects use the expected package manager by default without changing npm or yarn behavior.

Written by the indexing model from the issue text.