Feature: unused imports lint/fixer

[tzachshabtay]

I am getting a warning from protoc but not from the linter (using the vscode extension).

For example:

warning: Import google/protobuf/wrappers.proto but not used.

[yoheimuta]

@tzachshabtay Thank you for your suggestion. It sounds useful. I attached the label to this issue for starters.

[mayankbpatel]

Agree with @tzachshabtay. It would also be useful to identify unused imports (or unused messages in the case where all .proto files across the org are available in a central repo).