Cleanup/document source code access functions + add `filename()`

[c42f]

• Move all source code access functions which refer to source locations and strings into the top of source_files.jl, and add some documentation for these.
• Add filename() function to determine source file name of a syntax object
• Also add a minor generalization to SyntaxNode->Expr conversion code to make Expr conversion general enough to allow it to also be used for JuliaLowering.SyntaxTree. (internal/experimental interface, for now)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.09%. Comparing base (dbe9e5d) to head (c04ccc2). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #470 +/- ## ========================================== - Coverage 96.55% 96.09% -0.47% ========================================== Files 14 13 -1 Lines 4244 3990 -254 ========================================== - Hits 4098 3834 -264 - Misses 146 156 +10 ```

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