Add GraphQL v17 support

[ivome]

Summary

Adds GraphQL 17 support and simplifies hybrid CJS/ESM packaging by importing all graphql utilities from the package root.

Addresses #103

• GraphQL 17 compatibility: Handle getVariableValues() returning { variableValues } instead of { coerced }, use string-literal AST kind checks (GraphQL 17 makes Kind type-only), and relax error-message assertions for GraphQL 17's more verbose argument errors.
• Single-instance safety: Drop the deep graphql/execution/values import and all sed-based import rewriting in the hybrid-module scripts. Every graphql import now goes through the bare graphql specifier, which avoids mixing ESM/CJS copies of graphql in the same realm.
• Version support changes:
  • peerDependencies.graphql: ^16.6.0 || ^17.0.0 (drops 15.x and 16.0–16.5; getVariableValues/getArgumentValues became root exports in 16.6)
  • CI matrix: graphql ~16.6, ~16, ~17.0; Node 20 removed from CI
• Build: Add "DOM" to tsconfig.json lib so GraphQL 17's type definitions compile under TypeScript 4.4 (for AbortSignal/AbortController references).

Breaking changes

• Minimum supported graphql version is now 16.6.0 (was 15.0.0).
• Node 20 is no longer tested in CI.

Test plan

• yarn test passes on graphql 16.6.0, 16.14.2, and 17.0.2 (CJS + ESM)
• New test: @include(if: $show) with explicit variable values (covers variable → directive coercion across graphql versions)
• Built dist/ artifacts contain only bare graphql imports (no deep imports, no .mjs/.js rewrites)
• Isolated consumer smoke test against published dist/ on graphql 17 (no realm crash)