Add unused translation key check

[jamesmengo]

What are you adding in this PR?

Adds UnusedTranslationKey, a new Theme Check rule that reports default locale keys that are not statically referenced by the theme.

This checks:

• locales/*.default.json
• locales/*.default.schema.json

I'm only adding this to all config for now

What is covered / ignored

Example	What happens
{{ 'cart.title' | t }}	Protects cart.title as a literal storefront reference.
{{ 'cart.title' | translate }}	Protects cart.title; translate is handled the same as t.
"label": "t:sections.header.settings.title.label"	Protects sections.header.settings.title.label for schema locale files.
{{ 'cart.' | append: 'title' | t }}	Resolves the literal append chain and protects cart.title.
{{ 'title' | prepend: 'cart.' | t }}	Resolves the literal prepend chain and protects cart.title.
{{ 'products.product.' | append: state | t }}	Treats products.product. as a safe prefix and protects products.product.*.
{{ 'cart.items' | t: count: cart.item_count }}	Protects pluralization leaves such as cart.items.one and cart.items.other.
{{ translation_key | t }}	Treats the storefront reference as unbounded dynamic and disables storefront unused-key reporting.
{% assign key = 'actions.' | append: action %} + {{ key | t }}	Also treated as unbounded dynamic; this PR does not trace Liquid assignment/data flow.
shopify.sentence.words_connector in a locale file	Ignored by default; not reported as unused.
customer_accounts.order.title in a locale file	Ignored by default; not reported as unused. Reviewer callout: this default is worth checking.

What did you learn?

Resolving dynamic keys can be a much larger problem than I initially thought 😆. Oh, silly me.

Tophatting

• I added screenshots of the changes (before and after the changes if applicable)

Automated checks run:

• pnpm --dir packages/theme-check-common build
• pnpm --dir packages/theme-check-common type-check
• pnpm --dir packages/theme-check-node build
• pnpm exec changeset status --since=origin/main
• pnpm build currently fails before Theme Check on unrelated packages/prettier-plugin-liquid TypeScript errors
• pnpm test packages/theme-check-common/src/utils/translation-references.spec.ts packages/theme-check-common/src/checks/translation-key-exists/index.spec.ts packages/theme-check-common/src/checks/matching-translations/index.spec.ts packages/theme-check-common/src/checks/valid-schema-translations/index.spec.ts packages/theme-check-common/src/checks/unused-translation-key/index.spec.ts --run

Before you deploy

• This PR includes a new checks or changes the configuration of a check
  • I included a minor bump changeset
  • I ran the Theme Check build/config generator and confirmed there are no additional generated config changes
  • theme-app-extension.yml is not applicable here; generated config did not change
  • [Shopifolk] I've made a PR to update the shopify.dev theme check docs if applicable (link PR here).
• My feature is backward compatible

[jamesmengo]

@karreiro please sanity check this