Add config options to control URL behavior

[FredKSchott]

Changes

• New config: trailingSlash: 'always' | 'never' | 'ignore'; to resolve 💡 RFC: Trailing slashes #452
• New config: pageDirectoryUrl: boolean to resolve a secondary part of 💡 RFC: Trailing slashes #452
• 404.astro now outputs to 404.html instead of 404/index.html to work with Netlify, Vercel, etc.
• Some small cleanup around configuartion

Testing

• Tests added.

Docs

• Docs added.

[changeset-bot]

🦋 Changeset detected

Latest commit: 45a9981

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
astro	Patch

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astro-www – ./www

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[matthewp]

lgtm

[FredKSchott]

Would normally keep this open for a bit longer after the first LGTM, but merging now so that #1198 can be rebased correctly off of main.