Provide fastparse helpers for parser errors.

[PapyChacal]

Stacked on:

• Switch Optional to an orElse extension method. #194

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
core/src/main/scala-3/builtin/AttrParser.scala	71.42%	2 Missing ⚠️

Files with missing lines	Coverage Δ
core/src/main/scala-3/Parser.scala	84.70% <100.00%> (+0.58%)	⬆️
core/src/main/scala-3/builtin/AttrParser.scala	95.13% <71.42%> (-2.09%)	⬇️