Proof of concept for Transformers

[steverice]

This is a proof of concept for TypeSpec Transformers.

What it demonstrates:

• how transformers can be set up as part of libraries
• how transformers can be configured via tspconfig
• how transformers can be tested
• how transformers can make use of mutators

The basic structure here is:

• a library defines a "Transformer" that encapsulates all of its transform functionality (similar to "linter")
• each individual transformation is called a "Transform" (similar to "lint rule")
• libraries also define "Transform Sets" (similar to linter "rule sets") that bundle transforms into functional groupings