Create a code style guide

[dyladan]

We should have a style.md where we document all decisions pertaining to code style such as #1310 (comment). The linter is a good start, but linting can't solve every problem and there are always edge cases.

[legendecas]

Closing as https://github.com/open-telemetry/opentelemetry-js/blob/main/style.md has already landed.