refactor: improve parseBody function and docs

[msensys]

• Refactored parseBody and function to improve readability and maintainability.
• Simplified and clarified examples in JSDoc comments
• Improving performance from tests

[msensys]

I had some problems with Deno's process.env, but I fixed them

[yusukebe]

Is the below better to avoid using !?

  const contentType = headers.get('Content-Type')

  if (
    (contentType !== null && contentType.startsWith('multipart/form-data')) ||
    (contentType !== null && contentType.startsWith('application/x-www-form-urlencoded'))
  ) {
    return parseFormData<T>(request, options)
  }

[msensys]

Ready @yusukebe

[yusukebe]

LGTM!

[yusukebe]

@mgrithm

Thanks. Merging now.