Support #pragma once and refine codebase

[ChAoSUnItY]

Previously in PR #158, #ifndef is not sufficient enough to address optional file inclusion issue in shecc, since shecc currently includes file before tokenization / parsing pass. Therefore, #pragma once is the only way to prevent recursively file inclusion.

Although #pragma once is not standard anywhere in C standards, major C compilers such as GCC and Clang do support this preprocessor directive.

before #pragma once and codebase refinement:

after #pragma once and codebase refinement:

Notice for shecc contributors

This is a temporary workaround to enable contributors to have proper intellisense or other LSP suggestions available while developing shecc itself, which expects #pragma once to be removed in shecc source code when new C preprocessor is implemented.

Summary by Bito

This pull request implements `#pragma once` in several files to prevent recursive file inclusion, enhancing the organization of the shecc compiler's codebase. It updates the lexer and parser to effectively handle `#pragma` directives, improving functionality and contributing to a more efficient development experience for contributors.

Unit tests added: False

Estimated effort to review (1-5, lower is better): 2

[jserv]

Rebase the latest master branch.

[jserv]

Move #pragma once right after the license notice.

[jserv]

It is a bit weird to place #pragma once in a non-header file.

[ChAoSUnItY]

I don't think current implementation (both MakeFile target script and shecc itself) is capable of linking source file to header file, in other words, we cannot declare global variables and function prototypes in globals.h and include it in other files. Also, even after migrating to header file, the source file would still need to add pragma since again, shecc is not capable of linking which requires to includes source and header file at once. This is workaround should be ignored by gcc when compiling, and is required for shecc in order to not included multiple times.

[DrXiao]

The proposed changes look good to me.

Due to the current limitations of shecc, it is still necessary for global.c to use #pragma once to prevent multiple inclusions.

We can resolve this once shecc is improved to handle more complex compilation in the future.
(If necessary, create an issue for tracking.)