Add CI for linting

[sffc]

Things we should cover in a linting CI:

1. Code format is consistent with cargo fmt
2. Copyright headers are correct

[sffc]

@echeran @zbraniecki

[zbraniecki]

Yep! Later we can consider clippy. Clippy has false-positives, so it requires some one-off cases to turn it off, but it generally helps avoid mistakes that cost perf/mem and the hints are ~90% useful in my experience.

[echeran]

How exactly should the copyright headers check go? Check that every file has copyright headers? Check that we're not adding code with copyright headers?

[sffc]

@hsivonen What is the correct strategy for license headers in code files in ICU4X?

[sffc]

Ping @hsivonen about copyright headers