Targa files (#2765) and JPEG XL images both open and place.
OPEN_EXTS and PLACE_EXTS list both, and UNASSOCIATED_EXTS keeps main's
four Targa extensions and adds jxl. The open_file row and the Glass
textures paragraph in docs/mcp.md, and the Glass's Load Texture line
in docs/target-app-parity.md, name both formats.