Add config to exclude files from routing

[tony19]

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---

Problem

The following directory layout creates an endpoint for post.spec.js and post.md, and the only way to opt out is to make them private modules with an underscore/dot prefix:

src
└── routes
    └── posts
        ├── post.md  👈 converted to endpoint!
        ├── post.spec.js  👈 converted to endpoint!
        └── post.svelte

Some developers might require the exact layout/naming shown, where:

1. The files are colocated
2. The files have the same basename

Solution

This PR adds kit.excludes config, which accepts an array of String, RegExp, or Function.

• String - The basename of the file/directory is compared against the string, and it's excluded if matches.
• RegExp - The basename of the file/directory is tested against the regular expression, and it's excluded if matches.
• ({ filepath, basename }) => boolean - The file/directory is excluded if the function returns true.

The default would be [^_.] to match the current behavior. This also would enable users to override the current behavior, allowing for underscore/dot-prefixed files to be included in routing.

Example config:

// svelte.config.js
export default {
	kit: {
		excludes: [
			/^[_.]/,
			/\.(test|spec)\.js$/,
			'README.md',
			({ basename }) => basename.startsWith('@'),
			({ filepath }) => filepath.includes('demo')
		]
	}
};

Closes #2930

[changeset-bot]

🦋 Changeset detected

Latest commit: 5933875

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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[Rich-Harris]

Thank you — am picking this up in #3576 (I couldn't push to this branch for some reason), so I'll close this