Add C++20 custom string literals for C64 PETSCII

LICENSE

@@ -232,3 +232,47 @@ such code will be identified clearly using at least one of two mechanisms:
    software, or
 2) It will contain specific license and restriction terms at the top of every
    file.
+
+Some of these licenses require their text to be included with distributions of
+the SDK. The text of these license is included below.
+
+==============================================================================
+UNICODE LICENSE V3
+
+COPYRIGHT AND PERMISSION NOTICE
+
+Copyright © 1991-2023 Unicode, Inc.
+
+NOTICE TO USER: Carefully read the following legal agreement. BY
+DOWNLOADING, INSTALLING, COPYING OR OTHERWISE USING DATA FILES, AND/OR
+SOFTWARE, YOU UNEQUIVOCALLY ACCEPT, AND AGREE TO BE BOUND BY, ALL OF THE
+TERMS AND CONDITIONS OF THIS AGREEMENT. IF YOU DO NOT AGREE, DO NOT
+DOWNLOAD, INSTALL, COPY, DISTRIBUTE OR USE THE DATA FILES OR SOFTWARE.
+
+Permission is hereby granted, free of charge, to any person obtaining a
+copy of data files and any associated documentation (the "Data Files") or
+software and any associated documentation (the "Software") to deal in the
+Data Files or Software without restriction, including without limitation
+the rights to use, copy, modify, merge, publish, distribute, and/or sell
+copies of the Data Files or Software, and to permit persons to whom the
+Data Files or Software are furnished to do so, provided that either (a)
+this copyright and permission notice appear with all copies of the Data
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+associated Documentation.
+
+THE DATA FILES AND SOFTWARE ARE PROVIDED "AS IS", WITHOUT WARRANTY OF ANY
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mos-platform/c64/CMakeLists.txt

@@ -9,6 +9,7 @@ install(FILES
   _sid.h
   _vic2.h
   c64.h
+  charset.h
 TYPE INCLUDE)
 install(FILES link.ld TYPE LIB)