Generalizing SourceFile - adding line/column offsets etc

https://github.com/JuliaDebug/Cthulhu.jl/pull/345 uses JuliaSyntax to represent source code with expression boundaries mapped to character positions within the source text. In that application, [CodeTracking](https://github.com/timholy/CodeTracking.jl) is used to extract source-text for specific methods which may be partway into a file.

Question: for "snippets" like these, should the starting line number `lineno` be added as a new field to `SourceFile`? Or do you need it to represent "a whole file"? If so, then adding it as a field seems incorrect, since of course a whole file will start at line 1. The other issue is whether functions like `source_line` should return the *offset* line or the line index; currently the two are the same. It's for reasons like these that I didn't want to add it without thinking about the consequences.

Read the linked Cthulhu.jl pull request first, then inspect the SourceFile abstraction and source_line behavior in JuliaSyntax. Determine whether snippets need line and column offsets and whether source_line should use offsets or indexes; done requires an agreed representation and documented semantics before implementation.

Written by the indexing model from the issue text.