Adding support for raygui (#95)

* updated build.zig and generate_functions.py to add raygui

* added raygui.h and manually coded some structs & enums in raygui prelude WIP

* ported all the structs, enums and globals to zig till raygui implementation

* imported types from raylib in raygui

* re-encoded raygui-prelude.zig. I don't know for some reason it was showing up as UTF-16 unicode text file. re-encoded it to UTF-8

* fixed imports in prelude to work properly with generated files

* updated generate_functions.py file to generate for raygui [text type error not fixed]

* simple temporary patch for mentioned text type issue

* removed unused imports from raylib in raygui

* added generated files

* Manually define raygui functions for slice arguments

* Manually define raygui functions with pointer return values

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Co-authored-by: Not-Nik <nik.wipper@gmx.de>

lib/preludes/raygui-ext-prelude.zig

@@ -0,0 +1 @@
+const rl = @import("raylib-zig");