fix: add type checking for optional properties

[Shubh-Raj]

Description

Add compile-time type checking for optional properties in TemplateMark templates. When an optional property is used without a guard ({{#optional}}, {{#if}}, or {{#with}}), the type-checker now throws an error at compile time instead of causing a runtime failure.

Before: Runtime error - "No values found for path '$['last']'"
After: Compile-time error - "Optional properties used without guards: last"

Changes

• Added validation in checkTypes() to detect unguarded optional property access
• Uses path-based scoping so guards only apply to descendant nodes
• Throws proper Error object with .errors property for structured access
• Added test case test/templates/bad/optional-noguard/

Testing

All 26 TemplateMarkInterpreter tests pass. All 33 snapshots pass.

Related Issues

Fixes accordproject/template-playground#11