fix(build): generate _routes.json based on public directory

[yusukebe]

Fixes #260

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[akku1139]

Thank you @yusukebe for working on #260.

I did some checks.

• Add/remove new files/directories to public/ and they will be added/removed to _routes.json .
• _routes.json is not regenerated by Vite if it is present in public/ .
• When I delete a files/directories from public/, they will also be removed from dist.
  build: _routes.json is not regenerated by emptyOutDir: false. #260 (comment)
  In the example below, fixing this can cause problems.
  https://zenn.dev/yusukebe/articles/06d9cc1714bfb7

The last one can be a problem when _worker.js uses env.ASSETS.fetch().

[yusukebe]

@akku1139

Can you write a test for this?

When I delete a files/directories from public/, they will also be removed from dist.