chore: add testing with Node.js 23

[trentm]

This was challenging because Node.js v23.6.0 enabled type-stripping
by default, which means node will attempt to execute .ts files by default.
This is incompatible with the .ts handling being used for running
tests (which was indirectly using ts-node for this via .mocharc.yml).

Instead, tests are now run by first compiling the test/** typescript
files to build/test/**, then executing them from there. This
required updating some relative paths to test files that were
not compiled (like .js fixtures, etc.).

Note: I didn't need to change any runtime code.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.00%. Comparing base (f9fda8b) to head (c40ef8c).

❗ There is a different number of reports uploaded between BASE (f9fda8b) and HEAD (c40ef8c). Click for more details.

HEAD has 4 uploads less than BASE

Flag	BASE (f9fda8b)	HEAD (c40ef8c)
	10	6

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #5410       +/-   ##
==========================================
- Coverage   94.65%   0.00%   -94.66%     
==========================================
  Files         317      23      -294     
  Lines        8025      40     -7985     
  Branches     1687       3     -1684     
==========================================
- Hits         7596       0     -7596     
+ Misses        429      40      -389     

see 305 files with indirect coverage changes