Enable defining custom entry points for LUAZIP apps at build time

[rdw-software]

No need to wait for an equivalent to package.lua when it can easily be stored as a new field in the LUAZIP signature.

Goals:

• If no entry point is given, default to main.lua (current behavior)
• If the LUAZIP version is not 1.0, expect a counted string (can be zero-length for "default")
• In that case, decode the entry point and load it instead of the default one
• Must ensure backwards-compatibility is guaranteed (can always load 1.0 AND new version)

Low priority as it complicates things and isn't really needed right now.

local vfs = {
	LUAZIP_MAGIC_VALUE = "LUAZIP",
	cdefs = [[
		#pragma pack(push, 0)
		typedef struct {
			char magicValue[6];
			uint8_t versionMajor;
			uint8_t versionMinor;
			size_t executableSize;
			size_t archiveSize;
			size_t customEntryPointLength;
			char* customEntryPoint; // Must interpret based on the previous field, if it's non-zero
		} lua_zip_signature_t;
		#pragma pack(pop)
	]],
}