Add SPR utilities

[pixeldesu]

The SPR (Spray/Sprite) file format is a custom scripting format defining particle animations.

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

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