Add SourceMap support

While generating source code, it would be very useful to know where which part of the output text came from.

The Twofold versions for Qt and .Net support source maps for this.

Unfortunately the `temple` gem does not support source maps for simplicity reasons.
At this point in time it's unclear what would be the best option to implements this and also if it's worth the effort.

Start by reviewing the existing Twofold source-map implementations for Qt and .Net, then inspect how the Ruby implementation uses the temple gem. Determine a feasible approach and whether the effort is justified; done means agreeing on a defined source-map design and implementing it for generated Ruby output.

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