Split `Unused*` rules into implementation and interface versions

[zaneduffield]

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 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.