feat: track reference origin nullability

[RomainMuller]

Allows emitting optional-chained references when the origin is conditional, so as to avoid the TypeScript compiler complaining about possibly null dereference.

[codecov]

Codecov Report

Merging #129 (30d754a) into main (169a091) will decrease coverage by 58.2%.
The diff coverage is 39.1%.

Additional details and impacted files

Components	Coverage Δ
Parser	39.5% <ø> (-57.8%)	⬇️
Intermediate Representation	38.8% <39.1%> (-50.6%)	⬇️
Synthesizers	32.4% <ø> (-55.6%)	⬇️
Other	17.4% <ø> (-57.9%)	⬇️

@@           Coverage Diff            @@
##            main    #129      +/-   ##
========================================
- Coverage   91.3%   33.1%   -58.2%     
========================================
  Files         22      22              
  Lines       2762    2820      +58     
  Branches       0    1340    +1340     
========================================
- Hits        2521     934    -1587     
- Misses       241     893     +652     
- Partials       0     993     +993     

Impacted Files	Coverage Δ
src/errors/mod.rs	16.7% <ø> (-25.0%)	⬇️
src/ir/conditions.rs	27.2% <ø> (-45.4%)	⬇️
src/ir/resources.rs	38.3% <16.7%> (-51.4%)	⬇️
src/ir/reference.rs	39.2% <42.9%> (-37.1%)	⬇️
src/ir/mod.rs	56.1% <64.3%> (-41.3%)	⬇️

... and 16 files with indirect coverage changes

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[RomainMuller]

NOTE - Coverage now is branch-aware, and Coverall reports "fully covered" only, so this is a more accurate view, although perhaps a bit scary