feat(plugins/fs-routes): allow to customize fresh app tree and exts

[JOTSR]

Currently fresh enforce usage of routes and islands as respectively "routes" and "islands" directory.
Also fresh only support js(x) and ts(x) files extensions to registers routes.

I currently facing a problem using fresh to build project to render mdx files with a custom semantic.
Despite options to overload "island" and "route" loading there is no options to define other roots.
This PR allow to customize the default behavior of routes and islands registrations.

For example:

await fsRoutes(app, {
    dir: './',
    loadIsland: (path) => import(`./islands/${path}`),
    loadRoute: async (path) => {
        const { default: Page, frontmatter } = path.endsWith('.mdx')
            ? await compileMdx(`./pages/${path}`)
            : await import(`./pages/${path}`)
    
        // Define Page component
        // Pass PageProps and allow custom props
	const Component =
            (pageProps: PageProps) => (props: Record<string, unknown>) =>
            Page({ ...pageProps, ...props })

        // Define App Wrapper
        const Wrapper = (props: PageProps) =>
            Template({ ...props, Component: Component(props), frontmatter })

        // Load wrapped route
        return { default: Wrapper }
    },
    routesDir: 'pages',
    fileExts: ['mdx'],
})

This allow the following directory structure:

Mode  Size Name
d----    -   .
d----    -  ├──  components
-a---  146  │   ├──  Authors.tsx
-----  232  │   ├──  Button.tsx
-----  492  │   ├──  Katex.tsx
-----  182  │   ├──  PapexBadge.tsx
-a---  653  │   └──  Raw.tsx
-a---  153  ├──  config.ts
-a--- 1.2k  ├──  deno.json
d----    -  ├──  islands
-----  361  │   ├──  Counter.tsx
-a---  769  │   └──  Plotly.tsx
----- 1.9k  ├──  main.ts
d----    -  ├──  pages
----- 1.1k  │   └──  main.mdx
-a---  381  └──  README.md