Support throw expressions

Summary

Implement the Stage 2 TC39 proposal: https://github.com/tc39/proposal-throw-expressions

Implement the current throw-expression grammar, precedence, early errors, single operand evaluation, and thrown completion propagation.

Why

This proposal fits GocciaScript's modern ECMAScript surface and the 0.13.0 proposal roadmap. It should extend the engine's existing architecture rather than arrive as a userland shim.

Current behavior

throw is only a statement, forcing block bodies or helpers in expression branches.

Expected behavior

Implement the current throw-expression grammar, precedence, early errors, single operand evaluation, and thrown completion propagation.

Scope notes

Reuse existing thrown-completion machinery and target bytecode after #875. Verify arrows, conditional/logical expressions, parameters, nested forms, and existing throw statements. Additive syntax: no flag.

Related bytecode roadmap: #871 and #875. Update the relevant language/built-in documentation and Goccia proposal inventory. Use the proposal repository and its current specification at implementation time; add focused project tests and matching pinned Test262/proposal coverage without regressions.

Acceptance criteria

• Parser/API surface, descriptors, evaluation order, errors, and observable protocol behavior match the current proposal.
• Implementation uses the identified shared engine seam without a duplicate compatibility shim.
• Focused positive, negative, abrupt-completion, and edge-case tests cover the proposal.
• Relevant project suites, pinned Test262 cases, formatting, and documentation checks pass.

Start with the parser/API surface and the shared thrown-completion machinery, then inspect the bytecode work tracked by #875. Use the current TC39 proposal to define grammar, precedence, early errors, and evaluation behavior; add focused project tests and pinned Test262/proposal coverage, with relevant documentation updates, until the listed suites and checks pass.