fix: add `fs/promises: false` to the browser config in package.json to exclude it from browser builds

[samwillis]

Should fix #355

Regression was introduced here https://github.com/electric-sql/pglite/commit/2831c3401ae146702106c64a8bb25f2bbcfd8b14 with the use of const fs = await import('fs/promises')

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Built bundles:

• Postgres WASM: https://github.com/electric-sql/pglite/actions/runs/11102842618/artifacts/1994501084
• Interim build files: https://github.com/electric-sql/pglite/actions/runs/11102842618/artifacts/1994501176
• PGlite: https://github.com/electric-sql/pglite/actions/runs/11102842618/artifacts/1994528527
• Demos: https://github.com/electric-sql/pglite/actions/runs/11102842618/artifacts/1994543150

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