Add evaluator with environment and file-based execution

Implements recursive AST evaluator with variable environment,
reads .j files from command line args, and executes programs
end-to-end (lexer -> parser -> eval).

projects/simple.j

@@ -1 +1 @@
-x = 1
+x = 1

projects/sum.j

@@ -0,0 +1,4 @@
+x = 10
+y = 5
+z = x + y 
+print z