test: add comprehensive e2e test suite for ZynML

Grammar Compilation Tests:
- LanguageGrammar compilation
- Grammar2 compilation and metadata verification

Parsing Tests:
- Basic syntax: let bindings, functions, parameters
- Operators: arithmetic, comparison, unary, pipe, matrix multiply
- Control flow: if, while, for loops
- ML constructs: tensor literals, method calls, chained methods
- Imports and comments

Example File Tests:
- hello.zynml, basic_tensor.zynml
- audio_pipeline.zynml, vector_search.zynml
- Graceful handling of unsupported syntax (abstract_types.zynml)

Stdlib Tests:
- prelude.zynml parsing
- tensor.zynml with advanced syntax detection

Runtime Tests:
- Runtime creation with/without plugins
- Grammar2 availability check
- JSON parsing via runtime
- Plugin loading verification

Execution Tests:
- Simple expression execution
- Example file execution with panic handling

Error Handling Tests:
- Unclosed braces
- Invalid syntax
- Unmatched parentheses

Author: darmie