Add template for SPA Mode

[brophdawg11]

Depends on #8338

Adds a template for SPA mode

Eventually:

npx create-remix --template remix-run/remix/templates/spa

Right now, to test:

npx create-remix@0.0.0-experimental-83eedbbcd --template https://github.com/remix-run/remix/tree/spa-mode-template/templates/spa

[changeset-bot]

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Latest commit: 1d7e1bb

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