ci: settle on consistent style and add warnings for violation

[jackgerrits]

I would like to propose we change the preferred type name style and then eventually enforce this in CI.

I propose we change from the most common style of snake_case for type names to PascalCase, and enforce snake_case for namespaces. Once the changes are made we can promote this clang-tidy warning into an error.

NOTE: functions and variables remain as snake_case

The primary motivator of changing types from snake_case to PascalCase is it allows variables and types to have separate style. Often it makes sense to name a variable similar to the type. e.g. vw::Example example; vs vw::example example;

Additionally, requires enum values and macros be UPPER_CASE.

See here for a report detailing changes required so we adhere to these checks:
https://gist.github.com/jackgerrits/c8da9db95457c372ad72c4fd72aff4b7#file-report-md

Resolution:
After discussion we will keep using the existing default of snake_case because there was opposition to change. Enforcing consistency will go ahead as planned.

[olgavrou]

I like this as long as we don't introduce a breaking change in between minor releases (discussed offline, breaking change can be pushed back with aliasing)

[jackgerrits]

100% agree on avoiding breaking changes here. Types which are considered part of the public API surface will have the necessary using x_old = x_new; put in place with a deprecated marker to avoid breaking changes and make it easier to migrate.