Fix CLI template gitignore scaffolding

[cameronapak]

Fixes #11342

This fix makes it where .gitignore is properly added to the scaffolded project when running npx remix@next new.

Why does the .gitignore file not get added when scaffolding a Remix 3 project? NPM packlist says, "If you have a package.json file with a files array within, any top level .npmignore and .gitignore files will be ignored."

• Renames the root app template ignore file to template/gitignore so it survives npm packaging.
• Maps gitignore to .gitignore in the scaffold copy step.
• Extends the scaffold test to assert generated apps include .gitignore and do not leak a plain gitignore file.

This mirrors the established scaffolding convention used by Create React App templates: the template contains gitignore, and the create command renames it to .gitignore during project creation. npm's own packlist rules call out .gitignore and .npmignore as package-control files that are commonly omitted from package tarballs, and npm's package docs note that ignore files still affect package contents under included directories.

References:

• npm-packlist package file rules: https://github.com/npm/npm-packlist#readme
• npm package.json#files docs: https://docs.npmjs.com/cli/v11/configuring-npm/package-json/#files
• Create React App custom template docs: https://create-react-app.dev/docs/custom-templates/