Generic `sourcefile()` function

[c42f]

• Having a generic version of this is useful for other syntax tree types such as JuliaLowering.SyntaxTree
• Function to get the SourceFile for a syntax object
• Generic highlight() implementation in terms of this

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.65%. Comparing base (af768dd) to head (222d30c). Report is 2 commits behind head on main.

Files	Patch %	Lines
src/syntax_tree.jl	83.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #469 +/- ## ========================================== + Coverage 95.93% 96.65% +0.72% ========================================== Files 13 14 +1 Lines 3981 4251 +270 ========================================== + Hits 3819 4109 +290 + Misses 162 142 -20 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.