Rename `logger` in `@sentry/core` to `debug`

[AbhiPrasad]

Description

We should explicitly state that the logger exported from @sentry/core is the SDK debug logger.

This gives us a couple of benefits:

1. communicates intent of the export much better
2. avoids collision with logger exported from other sdks (which is used for sentry structured logging)
3. we can move structured logging logger code into @sentry/core, reducing duplication across our sdks

We'll first have to adjust the export in @sentry/core, and then change all the sdk code accordingly. After we change all the SDK code, we can deprecate the export.

[mydea]

while at it, we can also get rid of unused levels. Just leaving:

debug.log()
debug.warn()
debug.error()