Allow @ as label prefix

[tech-ticks]

Closes #52.

Note: I can't find a simple example to manually test this change with end-to-end, because simple code like

def 0 {
    @foo;
    jump @foo;
    end;
}

doesn't decompile to ExplorerScript and SSBScript only shows a jump to a "ghost label". However, this behavior is consistent with the current master (and an older SkyTemple version I have installed), so this change doesn't seem to break anything.