Add Support for Converting #define Constants to Go Constants (#469)

[ljluestc]

Title: Add Support for Converting #define Constants to Go Constants (#469)
Description:
This pull request addresses issue #469 by adding support for converting C #define constants (specifically T_EOF, T_SPACE, and T_TAB) into Go constants during the preprocessing stage of c4go. The change ensures these constants are preserved in the transpiled Go code, improving compatibility with C code that relies on such definitions.
Changes Made
Modified preprocessor/preprocessor.go:
Added a defines field (map[string]string) to the FilePP struct to store #define constants.

Updated NewFilePP to initialize the defines map and append Go-style const declarations to the preprocessed output (f.pp) when constants are present.

Modified analyzeFiles to take a *FilePP parameter, parse #define directives using a regex (#define\s+(\w+)\s+([^\s]+)), and store specific constants (T_EOF, T_SPACE, T_TAB) in f.defines.

Preserved the original include hierarchy logic to maintain existing functionality.

Added preprocessor/preprocessor_test.go:
Created unit tests to verify that #define constants are correctly converted to Go constants in the preprocessed output.

Tests cover both cases: files with #define constants and files without.

Why This Approach?
The constants are added at the top of the preprocessed output as a Go const block, making them available for later transpilation stages.

Limiting the initial implementation to T_EOF, T_SPACE, and T_TAB matches the issue’s example, but the framework is extensible for other constants.

Retaining the include hierarchy logic ensures no regression in existing preprocessor behavior.