Document potential byte-offset position 'hazard'

[acutmore]

Hi @branchseer, as the author of ts-blank-space, let me first say that this project is awesome to see! I had hoped that someone would demonstrate an even faster approach that avoided allocating the AST nodes, this wasn't an option for ts-blank-space due to its decision to use TypeScript's official parser API.

One aspect of the 'type-stripping' approach that is perhaps not immediately clear is that it not only preserves the line+column position but it also preserves the full string byte offset. This can be important because the code-coverage data produced by V8 is reported as byte-offsets, not line+column.

This means that when extra characters are added, even if the line+column is either preserved or very close to the original, the byte-offsets will incrementally get further and further away. This is one of the reasons why ts-blank-space doesn't support syntax such as the legacy type assertions <T>{} because it (sometimes) requires adding an extra closing ) after the }.

I wonder if it would make sense to mention this more explicitly in the README?

p.s. I also agree that in the future range mappings may be of big help here.