Squashed: platform support, bugfixes, GH workflows, read more

I've expanded support to three major platforms:

- linux
- windows
- macos

The CI now runs the test suites for all of these, both under normal
conditions and under GDB (except for macos, which lacks GDB support).

TOY_BITNESS specifies the bit-width of the current platform, either 32
or 64. A value of -1 means the bit-width could not be determined. Some
tests will be disabled if the appropriate bit-width can't be determined,
and a warning is printed to stderr.

TOY_API has been tweaked, and is now dependant on different
preprocessor flags. It is defined as 'extern' on all supported
platforms except windows, which instead specifies DLL support. It
defaults to 'extern' if the platform can't be determined.

commit d0350998ecc80b8925a1962ceb2ab400da50be9d
Author: Kayne Ruse <kayneruse@gmail.com>
Date:   Sun Sep 22 09:55:42 2024 +1000

    Expanded GDB tests using matrix strategy

commit dc2addacc52830227ddcd0f35997c0e1668b579c
Author: Kayne Ruse <kayneruse@gmail.com>
Date:   Sun Sep 22 09:05:42 2024 +1000

    Reserved the yield keyword

commit f485c380f74a49092e0c5a41e599fbb06dbce235
Author: Kayne Ruse <kayneruse@gmail.com>
Date:   Sat Sep 21 15:17:11 2024 +1000

    Potential segfault fix

commit d8b19d21c92133feb071e631009a3cf99df0f068
Author: Kayne Ruse <kayneruse@gmail.com>
Date:   Sat Sep 21 14:25:47 2024 +1000

    Added testing on windows under GDB, read more

    I'm hunting a segfault that only appears on windows, but I lack a
    windows machine, so github's runners are all I have right now.

commit 8606db541fb5cbe91b16a39e9815fe4a27ba0c8a
Author: Kayne Ruse <kayneruse@gmail.com>
Date:   Sat Sep 21 13:12:02 2024 +1000

    DLL import/export macros tweaked for windows

    TOY_EXPORT for making a DLL
    TOY_IMPORT for using a DLL

    Defaults to 'extern' if neither option is present

commit a6929666401953a5b3a93dfe83c9398e012beefc
Author: Kayne Ruse <kayneruse@gmail.com>
Date:   Sat Sep 21 12:52:06 2024 +1000

    Investigating bitness issue on windows

commit 8f615f735868a316e8d5a6a77ed899e72fd537f8
Author: Kayne Ruse <kayneruse@gmail.com>
Date:   Sat Sep 21 12:32:55 2024 +1000

    Adjusting bitness tests in test_ast.c

commit 61694f2183ac84ee7c53c855f2f6aa29f360f16c
Author: Kayne Ruse <kayneruse@gmail.com>
Date:   Sat Sep 21 11:46:59 2024 +1000

    Added experimental macOS CI job

source/toy_opcodes.h

@@ -36,5 +36,5 @@ typedef enum Toy_OpcodeType {
 	//meta instructions
 	TOY_OPCODE_PASS,
 	TOY_OPCODE_ERROR,
-	TOY_OPCODE_EOF,
+	TOY_OPCODE_EOF = 255,
 } Toy_OpcodeType;