feat(env): add optional key filter to parseEnv

[andrelandgraf]

Summary

• Adds a third way to call parseEnv: a typesafe key filter. parseEnv(config, ["DATABASE_URL", ...]) requires and returns only the selected OS-level env vars, instead of every variable the policy implies.
• Use case: a process that only uses a subset (e.g. a Next.js app reading DATABASE_URL but not DATABASE_URL_UNPOOLED) no longer throws over vars it never uses.
• The keys are typesafe via SelectableEnvKey<Config> — autocomplete only offers vars the policy enables, and selecting one from a disabled namespace is a compile error.
• The result type (FilteredNeonEnv<K>) is narrowed: it drops unselected namespaces and unselected properties within a kept namespace (selecting ["DATABASE_URL"] yields { postgres: { databaseUrl: string } }).
• Composes with the existing overloads — an array is unambiguous vs. the function-slug string at both runtime (Array.isArray) and the type level. Filter and function-slug scope remain separate, mutually-exclusive overloads.

Changes

• src/lib/env.ts: new overload + SelectableEnvKey / FilteredNeonEnv types + parseFilteredEnv runtime + FILTERABLE_ENV_KEYS reverse map.
• src/v1.ts: export SelectableEnvKey and FilteredNeonEnv.
• src/lib/env.test-d.ts (new): Vitest expectTypeOf type tests (positive equalities + @ts-expect-error negatives), plus a test:types script.
• src/lib/env.test.ts: runtime tests for the filter.
• tsdown.config.ts: exclude *.test-d.* from the published bundle.
• README.md + changeset (minor).

Test plan

• pnpm --filter @neondatabase/env tsc clean
• pnpm --filter @neondatabase/env test:ci (63 tests)
• pnpm --filter @neondatabase/env test:types (12 type tests, no type errors)
• pnpm --filter @neondatabase/env build — verified dist/ excludes test-d and expect-type