Release v1.0 of the spec.

Proposed changes:

• Mandate that the type-2 AppImage runtime must be statically linked so that it can run on non-glibc systems and will not require libraries such as libfuse2
• Mandate the use of the squashfs file format with zstandard compression for type-2 AppImages

References:

• Statically linked runtime AppImageKit#877
• Use zstd compression AppImageKit#478