feat(shopify-app-remix): allow overriding linkComponent on AppProvider

[SinhSinhAn]

What

Adds an optional linkComponent prop to <AppProvider> so apps can override the default RemixPolarisLink. When omitted, behaviour is identical to today (existing RemixPolarisLink is used), so this is a non-breaking minor.

import {NavLink} from '@remix-run/react';
import {AppProvider} from '@shopify/shopify-app-remix/react';

<AppProvider apiKey={apiKey} linkComponent={NavLink}>
  <Outlet />
</AppProvider>

Why

Closes Shopify/shopify-app-template-remix#486. The issue author (@blanklob) wants Polaris components that render links to use Remix's NavLink (or any custom wrapper) so they can pick up navigation state like isActive. Until now AppProvider Omit-stripped linkComponent from its public prop surface and hardcoded RemixPolarisLink, which left no escape hatch short of forking the provider.

This mirrors how i18n is already exposed: omit the Polaris prop, then re-declare a typed wrapper that defaults to the in-package value. The change is a single destructured default plus a new JSDoc-documented prop, so reviewers can audit it in one read.

What changed

File	Change
packages/apps/shopify-app-remix/src/react/components/AppProvider/AppProvider.tsx	Added linkComponent?: PolarisAppProviderProps['linkComponent'] to AppProviderProps with JSDoc. Destructured with RemixPolarisLink as the default. Forwarded to <PolarisAppProvider>.
packages/apps/shopify-app-remix/src/react/components/AppProvider/__tests__/AppProvider.test.tsx	Added one test verifying a consumer-supplied link component is forwarded to the Polaris provider (the existing test confirms the default still resolves to RemixPolarisLink).
.changeset/app-provider-custom-link-component.md	Minor changeset for @shopify/shopify-app-remix with a usage snippet.

Test plan

• All 6 existing AppProvider tests still pass (npx jest --testPathPatterns AppProvider)
• New regression test passes (custom linkComponent reaches <PolarisAppProvider>)
• Default-omit behaviour preserved (linkComponent: RemixPolarisLink assertion in the existing i18n test continues to pass unchanged)
• No type changes to the exported surface beyond adding an optional prop

Notes

• The issue author originally floated the name customLink, but linkComponent matches the existing Polaris convention (and is the prop we forward to anyway), so I went with that to avoid two names for the same thing. Happy to rename if maintainers prefer.
• The example uses Remix's NavLink, which is the headline use case from the issue's Loom.