Replace custom deepEqual with dequal/lite

[Pearce-Ropion]

Summary

• Replace the hand-rolled deepEqual utility with dequal/lite. The library is tiny (~300B) and correctly handles cases the custom implementation didn't (e.g. Date, RegExp, NaN, array length mismatches).
• Configure tsdown so dequal is external in the ESM and CJS outputs (consumers dedupe via their bundler) but bundled into the UMD build so the script-tag distribution stays self-contained.
• Drop the now-obsolete deepEqual.ts and its test file.

Implementation notes

• dequal is added to dependencies of @sigmacomputing/plugin, which makes tsdown externalize it by default in every format.
• packages/plugin-sdk/tsdown.config.ts overrides this for the UMD format via format.umd.deps.alwaysBundle, forcing dequal and dequal/lite to be inlined into the UMD bundle.

Verification

• yarn types passes.
• yarn build produces:
  • dist/esm/index.js → import { dequal } from "dequal/lite";
  • dist/cjs/index.cjs → require("dequal/lite")
  • dist/umd/sigmacomputing-plugin.umd.js → dequal inlined (no require('dequal'))

Test plan

• yarn install
• yarn build — inspect dist/esm/index.js, dist/cjs/index.cjs, and dist/umd/sigmacomputing-plugin.umd.js to confirm externalization vs. inlining.
• yarn test (browser tests via Playwright) — confirm useEditorPanelConfig still avoids re-configuring on equal inputs.
• yarn types

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