Add datasource BACKFILL skip support

[gnzjgo]

Summary

• Adds a backfill: "skip" option to defineDatasource.
• Emits BACKFILL skip in generated .datasource output when the option is set.
• Adds schema and generator coverage for the new option.

Fixes #161.

Reproducibility

Before this change, the issue example could not be represented by the SDK: adding backfill: "skip" to defineDatasource(...) was not part of DatasourceOptions, and generated .datasource output never included the BACKFILL skip directive.

This PR reproduces that path in tests by defining a datasource with:

defineDatasource("test_ds", {
  schema: { id: t.string() },
  backfill: "skip",
})

and asserting the generated .datasource content contains:

BACKFILL skip

Test plan

• pnpm exec vitest run src/schema/datasource.test.ts src/generator/datasource.test.ts
• pnpm exec tsc --noEmit