Add Nix flake

[jctemp]

Nix Flake Integration

Motivation

A Nix flake is introduced to streamline the development and building process of the Typst project.

Key Changes

1. Nix Flake Integration: Added a flake.nix file to define the development environment and build process. I adopted the Typst flake for that.
2. Testing Adjustments: Disabled tests requiring network connections due to Nix purity constraints.
3. Build Configuration: Updated to point to main.rs as specified in cargo.toml.

Testing and CSL Dependencies

• Current testing cannot be fully included in the flake setup due to CSL dependencies which are pulled during test phase.
  • An include with Nix flakes input would lock the version of CSL.
• So test requiring network connections are turned off to maintain flake purity and avoid lock.
• All tests can still be run in the dev shell and CI environments.

Build Configurations Tested

• [x] x86_64-linux
• [x] aarch64-linux
• [ ] x86_64-darwin (cannot test)
• [ ] aarch64-darwin (cannot test)

Note: Darwin should build as the Typst/Typst project can build, which depends on this project.