Split `Unused*` rules into implementation and interface versions

### Prerequisites

- [x] This improvement has not already been suggested.
- [x] This improvement should not be implemented as a separate rule.

### Rule to improve

Unused*

### Improvement description

Split each `Unused*` rule (e.g. `UnusedConstant`, `UnusedField`, etc.) into an implementation and interface variant.

### Rationale

Currently [integrated-application-development/delphilint](https://github.com/integrated-application-development/delphilint/) chooses not to run the `Unused*` rules in its analysis, because it's so likely to be inaccurate when only scanning a subset of the source files for a project.

If the rules were divided into 'public' and 'private' variants, then delphilint would be able to run the 'implementation' variants and confidently report unused local variables, constants, fields, methods, etc.