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1031bb65 - Formatter: Add SourceType to context to enable special formatting for stub files (#6331)

Formatter: Add SourceType to context to enable special formatting for stub files (#6331) **Summary** This adds the information whether we're in a .py python source file or in a .pyi stub file to enable people working on #5822 and related issues. I'm not completely happy with `Default` for something that depends on the input. **Test Plan** None, this is currently unused, i'm leaving this to first implementation of stub file specific formatting. --------- Co-authored-by: Micha Reiser <micha@reiser.io>