Add throw and try/catch/finally support

[memburg]

Goal

Add basic exception handling to GiavaScript.

Why

This enables safer application logic and controlled error recovery instead of only runtime-fatal behavior.

Scope

Implement support for:

• throw
• try { ... } catch (err) { ... }
• try { ... } finally { ... }
• try { ... } catch (err) { ... } finally { ... }

Plan

• Extend parser and statement handling for try/catch/finally and throw.
• Propagate thrown values through execution until caught.
• Bind catch parameter in catch scope.
• Add specs for normal flow, thrown errors, rethrow, and finally behavior.
• Update language reference docs.

Out of Scope

• Advanced engine-level stack traces in first pass.