[plugins] refactor framework setup into fresh plugins (closes #92)

[miguelrk]

Another take on https://github.com/netzo/netzo/issues/92.

Most importantly, this PR reverts the default project structure back to using the original dev.ts, main.ts and netzo.config.ts (extended fresh.config.ts) files of Fresh. This sacrifices DX in favor of aligning with fresh project structure, since improvements to the default project structure should best be done upstream in Fresh. We should help moving things forward over there instead...

For convenience, this PR introduces a netzo/plugins/mod.ts barrel file to bundle all netzo-specific plugins and allow import * as netzo pattern like bellow. One can always import individually. Standalone plugins like unocss are not in this bundle to make it clear they are netzo-independent.

// netzo.config.ts
import { defineConfig } from "netzo/mod.ts";
import * as netzo from "netzo/plugins/mod.ts";
import { unocss } from "netzo/plugins/unocss/plugin.ts";

export default defineConfig({
  plugins: [
    netzo.environments(),
    netzo.auth({...}),
    netzo.api({...}),
    unocss(),
  ],
});