perf: dynamic import dev server

[boehs]

closes #3621

cpuprof showed significant time spent loading libraries for the dev server. This ensures it is only imported when needed

~/Code/small/eleventyb: hyperfine "pnpm build" --warmup 1
Benchmark 1: pnpm build
  Time (mean ± σ):     636.3 ms ±  88.6 ms    [User: 572.3 ms, System: 92.2 ms]
  Range (min … max):   582.1 ms … 878.8 ms    10 runs
// after change
~/Code/small/eleventyb: hyperfine "pnpm build" --warmup 1
Benchmark 1: pnpm build
  Time (mean ± σ):     593.4 ms ±  49.5 ms    [User: 551.8 ms, System: 88.3 ms]
  Range (min … max):   533.4 ms … 701.4 ms    10 runs

tested by verifying eleventy --serve still works

assuming 50ms of savings, with 60k downloads per week, eleventy is likely called 100k times per week, or 5200000 times per year. This means this change will save 72 hours of compute per year! buying puts on AMD!!!!!

[zachleat]

LGTM, thank you very much!

[zachleat]

Shipping with v3.0.1-alpha.5