Allow author to prevent page overrides from users

[BryceRussell]

Currently, all pages inside a theme can be renamed or disabled by the user, for example:

package/
└── src/
    └── pages/
        ├── api/
        │   ├── login.astro
        │   └── logout.astro
        └── index.astro

// astro.config.mjs
import { defineConfig } from 'astro/config';
import myTheme from 'my-theme';

export default defineConfig({
  integrations: [
    myTheme({
      pages: {
		"/": "/home",
		"/logout": "/signout",
		"/login": false
      },
    }),
  ]
});

The purpose of this overriding it to prevent user's from being locked-in to the pages the theme creates. But in some cases, a theme may want to opt out of this overriding. ATP should provide a way to prevent users from overriding a page.

Things to consider:

• This should be discouraged! Themes are meant to be overridable, this should only be used for advanced use-cases where an override could break a theme
• Maybe the theme provider could allow overriding of .astro pages but not API route .js|.ts
• Users have two options when overriding a page: renaming and disabling. Can this API support the ability to rename, but not disable (and vice versa)?