💸 No spell-checker, so copy drifts between US and Australian English

[designbyalex]

Description

Nothing spell-checks this repo in CI. No cspell, vale or textlint config exists, so US and Australian English both sit in the tree.

Proposed solution

Add a spell-checker configured for en-US and run it in CI.

1. Decide whether to rename the Australian-spelled schema field names. Renaming them is a content migration, because every content JSON has to change with the schema or Tina content breaks.
2. Then add the checker, scoped to copy.