feat: dynamic file imports

[deer]

closes https://github.com/denoland/fresh/issues/1405 supersedes https://github.com/denoland/fresh/pull/1893

Trying this one again, since it seems pretty cool. Sadly there seems to be an issue with tests/fixture_layouts on my computer. I'm getting sporadic errors like this:

deno run -A main.ts
error: Top-level await promise never resolved
    "./routes/dynamic/[tenant]/index.tsx": await import(
                                           ^
    at <anonymous> (file:///Users/reed/code/denoland/fresh/tests/fixture_layouts/fresh.gen.ts:26:44)

But changing the deno runtime via deno upgrade causes it to go away before returning. Sometimes clearing the cache also seems to help.

[deer]

It's interesting that island in dynamic route test => ./tests/layouts_test.ts:163:6 consistently fails on windows and mac, and seems to be passing on linux.

[deer]

@iuioiua, perhaps you want to close your old one and we can go from here? This time I've hit all the manifests via deno task manifests (which I don't think existed when you last tried this).

[deer]

When I'm in fresh/tests/fixture_layouts and run deno run -A main.ts it always fails with:

error: Top-level await promise never resolved
    "./routes/dynamic/[tenant]/index.tsx": await import(
                                           ^
    at <anonymous> (file:///Users/reed/code/denoland/fresh/tests/fixture_layouts/fresh.gen.ts:26:44)

but when I run deno run -A dev.ts it always starts just fine.

[iuioiua]

Observation: when the ./routes/dynamic/[tenant]/index.tsx route is moved to be the first entry within the manifest.routes object, this issue does not occur. It seems we're encountering some race condition. Either way, I suspect this to be a fault of the runtime.