Add exports map with ESM-flavoured type declarations for node16/nodenext

[markedwards]

Problem

Under TypeScript's moduleResolution: node16 / nodenext, material-react-table resolves to the wrong type shape. Types-only cause: the package has no exports field, and its single dist/index.d.ts is read as CommonJS when the package has no "type": "module" — regardless of which condition resolved it — so ESM consumers get CommonJS-shaped declarations and named imports/types resolve incorrectly.

Fix

Add an exports map that routes types per condition, and emit an ESM-flavoured dist/index.d.mts. Runtime resolution is deliberately unchanged:

"exports": {
  ".": {
    "module":  "./dist/index.esm.js",                                  // bundlers → ESM (matches the existing `module` field)
    "import":  { "types": "./dist/index.d.mts", "default": "./dist/index.js" },
    "require": { "types": "./dist/index.d.ts",  "default": "./dist/index.js" }
  },
  "./*": "./*"
}

• Node's import and require both resolve to the CJS build (dist/index.js) — exactly what main resolved to before, so there is no runtime change for Node consumers.
• The module condition (honoured by webpack/Vite/etc., ignored by Node) keeps the tree-shakeable ESM build for bundlers, matching the pre-existing module field. Bundlers that don't honour it fall back to import → CJS.

dist/index.d.mts is produced as a second output of the existing rollup-plugin-dts step — identical bundled declarations, no copy step, no new dependency. main/module/typings are retained and "./*" preserves deep imports (e.g. locales), so the change is additive.

Compatibility — please review before choosing a release type

Introducing an exports map restricts the package to its declared entry points. "./*": "./*" preserves arbitrary subpaths (dist/*, src/*, package.json, and any file imported with an extension), so the common surfaces are unaffected. One raw-Node pattern does change:

• require('material-react-table/locales/<lang>') resolved to locales/<lang>/index.js before (CJS directory resolution via each locale's own package.json); with the exports map it becomes ERR_PACKAGE_PATH_NOT_EXPORTED, because exports patterns don't perform directory/index resolution. Bundler consumers (webpack/Vite/Next/etc.) are unaffected — they directory-resolve the subpath themselves — and ESM import of an extensionless locale subpath was already unsupported. So this only affects code doing a raw-Node require() of a locale directory.

If you'd like to preserve that path (and keep this non-breaking), add a locale subpath entry — verified to restore both require and import of locales/<lang> to the pre-exports target:

"./locales/*": {
  "module":  "./locales/*/index.esm.js",
  "import":  { "types": "./locales/*/index.esm.d.ts", "default": "./locales/*/index.js" },
  "require": { "types": "./locales/*/index.d.ts",  "default": "./locales/*/index.js" }
}

I left it out of this PR to keep it focused on the type-resolution fix — happy to add it if you'd prefer. Either way, the semver/release decision is yours.

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