Release v1.5.0

The full invoicing .play sample had drifted behind the language and
only exercised a subset of the available constructs. It is now a
complete, idiomatic showcase of the current Screenplay language, kept
identical between the Monaco editor sample and the compiler fixture so
the editor's default content is exactly what the compiler and printer
round-trip tests validate.

Base branch: this targets feature/multiline-descriptions rather than
main because the sample showcases the fenced multi-line descriptions
and command descriptions that only exist on that branch. It follows to
main once that branch merges.

Added

• Editor syntax highlighting for the specification, given, then and error keywords.
• Editor syntax highlighting for the remaining projection keywords (sequence, all, nested, automap, no, exclude, clear, via, literal) and capture keywords (split, nested, from, to, and, or, tag).
• Editor syntax highlighting for the $eventSourceId and $causedBy context variables.

Changed

• The full invoicing sample now exercises every language construct at least once: the @sensitive concept attribute, the complete set of concept and command validation rules, claim-based and grouped authorization policies, a nested feature, file-based handlers, the full projection sub-language (sequence, every/all, composite keys, nested blocks, remove via join, set/count/add/subtract mappings and the $eventSourceId/$causedBy/literal/template expressions), the remaining capture triggers (split, value transition, logical and/or, template), screen titles/summaries/file references, html and typescript code blocks, multiple reactor triggers and event-based specifications.