Add plpgsql_check extension support

[racinette]

Adds plpgsql_check (PL/pgSQL linter and static analyzer) as an external PGlite extension, distributed as @electric-sql/pglite-plpgsql-check.

Companion backend PR: electric-sql/postgres-pglite#80 — must merge first; the postgres-pglite gitlink here points at that branch, so CI cannot resolve the submodule until it lands (happy to re-point at the merged SHA then).

• New package packages/pglite-plpgsql-check (follows the pgmq/pgtap pattern), with tests covering: extension load, invalid/valid function diagnosis, dump/load round trip, multiple instances, transactional rollback, and schema-qualified regprocedure signatures — 7/7 passing locally
• postgres-pglite submodule bump: builds the extension tarball and exports the core symbols it needs
• CI: cache/upload/download/dist handling for the new package in build_and_test.yml
• Docs: extensions catalog entry, REPL export, docs deps
• Changesets: initial release for the new package + patch for @electric-sql/pglite (new WASM exports)

Local verification: full core suite green (56 files / 279 tests), lint/typecheck/attw clean, docs site builds.