Write NSFe format

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The NSFe format allows including more structured metadata than NSF. It begins with four bytes NSFE in ASCII, followed by a sequence of chunks as length, fourcc, data tuples: "INFO" first and "NEND" last. Integers are little-endian as in NSF, and no CRC is used unlike PNG.

NSFPlay and VLC media player can reportedly play NSFe files.

• [x] Get it writing a valid NSFe with the essential chunks: INFO, DATA, and NEND
• [x] If PENTLY_USE_PAL_ADJUST, enable Dendy mode in the player by writing a regn chunk containing 07 00
• [x] Add auth chunk containing "$title\x00$author\x00$publisher\x00"

After #38, add credit metadata:

• [x] Fix auth chunk not to hardcode the strings
• [x] Write song titles as null-terminated strings in tlbl chunk
• [x] Write song authors as null-terminated strings in taut chunk
• [ ] If PENTLY_USE_NSF_SOUND_FX, write a psfx chunk containing all bytes from NUM_SONGS through NUM_SONGS + NUM_SOUNDS - 1.

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Basic NSFe generation is in. But because the length of psfx depends on NUM_SOUNDS yet must be determined at assembly time, adding psfx will have to wait for include file generation, which I plan to add along with #38.

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Verifying the last part (psfx) depends on an NSFPlay defect fixed in b100dfc. Will close once that fix lands in the next beta release and I can test.

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NSFPlay 2.4b13 fixes psfx parsing, reading the NSFe correctly.