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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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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