[WIP] Custom rule AST matching/querying

[aaroncrespo]

JP and Keith both expressed some interest so I figured I would submit early as I work to resolve some of the TODO's and FIXME's and get the ball rolling

The goal with this change is to write a rule similar to a regex rule but structured.

We had a time when we wanted to match log(.w, "Could not ...\(logContent)" but not log(.w, scrubbed: "Could not ...\(logContent)".

You can imagine that since anything and any whitespace could occur within the interpolation writing a regex for this is not straight forward.

[SwiftLintBot]

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* [WIP] Custom rule AST matching/querying.  
  [aaroncrespo](https://github.com/aaroncrespo)
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