CLI Modifications

We have changed the following parts of the CLI (crate compiler-cli):

1. new.rs (glistix new):
   • Added Nix-relevant file templates (default.nix, shell.nix and flake.nix);
   • Clone glistix/stdlib to externals/stdlib as a Git submodule by default;
   • Changed default gleam.toml to include Glistix-specific options.
2. run.rs (glistix run, glistix test):
   • Use nix-instantiate when calling glistix run or glistix test on the Nix target.
3. publish.rs (glistix publish):
   • Implement [glistix.preview.hex-patch] by replacing dependencies with what's specified in hex-patch right before publishing.
4. dependency.rs (resolving versions for the manifest.toml):
   • Implement local-overrides from [glistix.preview] by replacing provided (local/Git) dependencies with what the root package specified for them, if overridden by the root.
5. fs.rs: Added Git operations used by new.rs.

glistix build (build.rs) wasn't directly modified, but it now supports --target nix as well.