Added external functions and object compilation

[mealet]

🍀 Description

Version: v1.1.3
Related: #31

1. Removed -l short flag for LLVM IR compilation (now only --llvm)
2. Flag -i --include replaced with -l --link
3. Added --object flag to compile source to object file (semantical main module checkers are disabled)
4. Flag --llvm now avoids main module checkers
5. Added ext visibility keyword to mark functions as external (this will add external linkage and avoid mangle)

Example:

ext fn add(a: i32, b: i32) i32 {
  return a + b;
}