fix: prevent race condition in tar extraction with install marker

[abhu85]

Summary

Fixes #50 - Adds an installation marker file to ensure tar extraction completed successfully before returning.

Problem

The previous implementation only checked for the presence of DynamoDBLocal.jar to determine if installation was complete. This caused intermittent failures when:

1. A previous download/extraction was interrupted (network timeout, process exit, slow disk)
2. The jar file exists but DynamoDBLocal_lib/ directory is incomplete
3. Tar stream events fired before disk writes fully completed

Users reported errors like "Invalid or corrupt jarfile DynamoDBLocal.jar" (#30) and "Unable to initialize main class" (#38) due to incomplete installations being used.

Solution

1. Install marker file: Creates .dynamodb-local-install-complete only after extraction is verified successful
2. Dual check: Checks for both jar file AND marker before skipping installation
3. Clean reinstall: Removes incomplete installations before re-downloading
4. Verification: Confirms jar file exists before creating the marker

Changes

• Added INSTALL_MARKER constant for the marker filename
• Updated installDynamoDbLocal() to check for both jar and marker
• Added cleanup of incomplete installations with fs.rmSync()
• Added onExtractionComplete() handler that verifies and creates marker
• Uses { recursive: true } for directory creation

Test plan

• Module loads successfully
• Syntax validation passes
• Maintains backward compatibility (first run will re-download, subsequent runs use marker)

Notes

Existing users will re-download DynamoDB Local once after this update (since marker file won't exist), but this is a one-time cost that ensures installation integrity.