Add CI copyright headers check

[echeran]

This task is split off from #84 to represent just the requirement of doing checks on copyright headers on source files.

We want every file to have headers indicating that indicates that the file is a part of ICU4X and references a link to the license.

Anything else I'm missing? @hsivonen

[hsivonen]

I created a PR that updates CONTRIBUTING.md to document the license header language that was discussed.

For CI, this means checking that .rs files start with

// This file is part of ICU4X. For terms of use, please see the file
// called LICENSE at the top level of the ICU4X source tree
// (online at: https://github.com/unicode-org/icu4x/blob/master/LICENSE ).

and .toml files start with

# This file is part of ICU4X. For terms of use, please see the file
# called LICENSE at the top level of the ICU4X source tree
# (online at: https://github.com/unicode-org/icu4x/blob/master/LICENSE ).