LFortran: Finish AST generation

This project aims to ensure that all the grammar rules defined are exposed to the AST(Abstract Syntax Tree) level.
‣ Basically working on Lexical and Syntax Analysis (Two phases of the Compiler Design).
‣ Some grammar rules were not defined in the parser. After adding them, created AST nodes related to those grammar rules, defined macros, used them in the parser to expose those nodes to the AST level.
‣ Parse Comments, Semicolons, and Newlines in the Fortran code. Preserve them in the AST, and print them using fmt(Subcommand).
‣ Add tests for each implementation to make sure everything works correctly as expected.