Compiler: Add early error for output path being a directory.

- Introduce new `Path` type and an array of build paths on the build context.
- Resolve input and output paths/files early (before parsing).
- Error early if inputs are missing or outputs are directories.
- Plumb new file path generation into linker stage instead of its adhoc method.

TODO:
- Remove more adhoc file path generation in parser and linker stage.
- Make intermediate object file generation use new path system.
- Round out and robustify Path helper functions.

src/llvm_backend_general.cpp

@@ -87,7 +87,6 @@ bool lb_init_generator(lbGenerator *gen, Checker *c) {
 		return false;
 	}
 
-
 	String init_fullpath = c->parser->init_fullpath;
 
 	if (build_context.out_filepath.len == 0) {