Add a Nix flake for source-based installs

[y0usaf]

Summary

• add a flake.nix + flake.lock for Nix users
• package linear from source instead of pinning per-platform release binaries
• run GraphQL codegen during the Nix build and install a wrapper that runs from the packaged source tree
• switch GraphQL codegen to useTypeImports so the generated code does not try to import the typings-only @graphql-typed-document-node/core package at runtime

Notes

• this uses aMOPel/deno2nix's custom-made-fetcher helper to materialize Deno/npm/jsr deps from deno.lock
• the package is source-based, so updates only need the single denoDepsHash refreshed instead of per-platform asset hashes
• verified with nix build path:.#linear and ./result/bin/linear --version