Move `WikiMarkupTransformer` outside of `parser` sub-package

[Chris Fuller]

Putting it in there was an accident attributable to how the code was developed and the fact that it merges two of the source files from different packages in the original TypeScript. Given that it wraps both the parser and the encoder, it belongs in the top-level package.

Plan:

• 0.15.1 – Refactor/rename up one package level, then restore original with @Deprecated markings
• 0.16.0 – Remove the original

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[Chris Fuller]

Fixed as was planned:

• 0.15.1 – Refactor/rename up one package level, then restore original with @Deprecated markings
• 0.16.0 – Remove the original