[process] Establish Java/Kotlin coding standards

[pq]

For Dart contributions, we can point folks to the Dart style guide as a North star (while flutter has it's own). We should establish what we want to do for Java/Kotlin and communicate that in the PR template and Contributing guides as well. (See: #8062).

See also: https://github.com/microsoft/vscode/wiki/Coding-Guidelines