[Bug] Making the library size become 0.45MB, instead of the current 4.5MB using a simple refactor

[fzyzcjy]

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Describe the bug
Thanks for this convenient library! This lib seems to depend on SwiftProtobuf, but when checking the app size report, I find SwiftProtobuf.framework takes 4.1MB (if uncompressed). The barcode_scan itself is actually only 0.3MB, and MTBBarcodeScanner is 0.15MB. Therefore, any user who uses this wonderful lib will have to pay for 4.55MB space even if the lib is merely 0.45MB - a 10x waste thing :(

Thus, I would appreciate it if the lib could be refactored without using protobuf. Actually, Flutter has officially released the pigeon package. I have used it in my app and it looks very great. Using it, there is no need for SwiftProtobuf any more.

To Reproduce
Steps to reproduce the behavior:
N/A

Expected behavior
Not contain SwiftProtobuf

Screenshots
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Logs
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Environment (please complete the following information):

• iOS? Version: latest
• Android? Version: ...
• Real Device? Which?: ...
• Emulator/Simulator? Which?: ...
• Version of barcode_scan: latest
• Output of flutter --version: latest

Flutter ...

Additional context
Add any other context about the problem here.