Fix type checking and codegen bugs, add Docker and test suite

[angelosSfyr]

Type checking:

• Reject duplicate classes, fields, methods, parameters and local variables
• Allow returning a subclass of the declared return type
• Check local variable and field types, println arguments and integer literal range
• Report undefined identifiers and use of main's argument instead of crashing
• Compare strings with equals() instead of ==

Code generation:

• Support boolean array allocation and convert boolean elements to/from i32
• Emit a vtable for the main class; main no longer takes a vtable slot

Main exits with status 1 on errors and only writes the .ll file after type checking.

Add a Dockerfile, run_tests.sh (compares compiled programs against javac/java), a GitHub Actions workflow, and an updated README.