…methods/properties/events)

Analyzer flags two cases: members with no read/writes and members with only writes.

1. Members with no read or writes: `Type '{0}' has an unused private member '{1}' which can be removed.`
2. Only writes: `Type '{0}' has a private member '{1}' which can be removed as the value assigned to it is never used.`

Code fix removes the unused member declaration.
Fixes #24225

Open questions:
1. Current analyzer design uses a single code style option for all members and both the above kinds of unused members. We can potentially have multiple options, but this should probably be done based on feedback.
2. Should the analyzer use different diagnostic IDs for the above two kinds of unused members? This will mean that the FixAll experience will need multiple iterations for removing unused members.
3. Should we update the code fix (or have an additional code fix) that also updates the write references for the case (2) above? Or is it better to leave the references to break the code so the user can analyze if the value being assigned can also be removed. The PR current chooses the latter approach as it is more conservative and unlikely to cause silent breaks.