Add SEQ utilities

[pixeldesu]

The SEQ (Sequence) file format is a custom scripting format defining particle animation sequences (calling SPR files, basically).

It shares the same format as SPR files, with the difference that SEQ files can contain a frame/ms count in front of a command.

Examples and a reference can be found at https://docs.pangya.golf/pc/file-formats/seq

Write up PangLib.SEQ.SEQFile parsing the file and returning an AST, which can then be used by other tools to execute the given commands with their arguments.