Extend from an outside directory

Add header and footer content, blocks, collections, CMS editors and routes to one or many sites from a directory that lives outside their repositories.

Create the directory and add the files you need from the table below.
Point STOMME_SLOTS_DIR at it when you run the site.
```
STOMME_SLOTS_DIR=../extensions pnpm dev
```
Read the log. The routes line names each slot that was found as slot:<name>, plus every route the directory added, and stomme cms lists the blocks it added as addon blocks.

A file that is not there changes nothing, and without STOMME_SLOTS_DIR the site builds exactly as it would with none of them.

File	What it adds
`header-start.astro`, `header-nav-end.astro`, `header-end.astro`	markup at that place in the header
`footer-end.astro`, `footer-legal-end.astro`	markup at that place in the footer
`blocks.mjs`	`export const blocks = { type: Component }`, components the renderer dispatches by type. The engine's block of the same type gives way to yours, and the site's own registry wins over both
`block-catalog.mjs`	`export const BLOCKS = [...]`, catalog entries for those blocks; a type the site's own catalog already has is skipped. `stomme cms` warns about a file it cannot read and goes on without its blocks
`collections.mjs`	`export const collections = { … }`, content collections added beside the engine's. They reach the site only when its `src/content.config.ts` spreads `...stommeAddonCollections()` from `@gronare/stomme/addon-collections` into its `collections`
`cms.mjs`	`export const collections`, a list of `{ feature, yaml }` entries or a function returning one: CMS editors added when that feature flag is on. The function is called with `{ routes, features, blocks, fields }`: the site's routes and feature flags, `blocks(indent)` returning the YAML of the site's block list, and `fields.button(name, label, indent = 8, hint)` and `fields.link(name, label, indent = 8, hint)` returning the YAML of a button or a link field. `panelFiles`, an object or a function of the same argument, maps an existing collection's name to `{ feature, yaml }` files added to it the same way
`routes.mjs`	`export const routes`, a list or a function of `{ routes, features }` returning `{ feature, pattern, entrypoint }`: a route injected when that feature flag is on, and under every language prefix on a translated site
`preview.astro`	what `/preview?kind=<kind>` renders for a kind the engine does not know, so your CMS editors can preview their own entries. It gets the props `kind`, `draft` (the entry being edited) and `config` (the resolved site config)
`previews.js`	CMS preview templates, copied to `public/admin/stomme-addon-previews.js` and loaded by `/admin`

An entry in cms.mjs without a feature or a yaml, or in routes.mjs without a feature, a pattern starting with / or an existing entrypoint, is skipped with a warning. A cms.mjs or routes.mjs that fails to load stops the build with its path.