support changing config after initdb, or editing from user call

[pmp-p]

nb : does not handle modifying arrays, they are overwritten.

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• Postgres WASM: https://github.com/electric-sql/pglite/actions/runs/11800927585/artifacts/2177178615
• Interim build files: https://github.com/electric-sql/pglite/actions/runs/11800927585/artifacts/2177178725
• PGlite: https://github.com/electric-sql/pglite/actions/runs/11800927585/artifacts/2177264193
• Demos: https://github.com/electric-sql/pglite/actions/runs/11800927585/artifacts/2177286911

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