Enable concurrent downloads on brew bundle

[BasixKOR]

Verification

• This issue's title and/or description do not reference a single formula e.g. brew install wget. If they do, open an issue at https://github.com/Homebrew/homebrew-core/issues/new/choose instead.

Provide a detailed description of the proposed feature

Related: #18278

As far as I can see the brew bundle command still works the old way: fetching one at a time. I couldn't find a issue tracking this and unsure if this was left out due to the scope so I opened this issue to track the current status.

What is the motivation for the feature?

Brewfile often contains much more formulae/casks than a typical brew install does so this could masively improve the installation speed for Brewfile users.

How will the feature be relevant to at least 90% of Homebrew users?

It won't affect the users doesn't using brew bundle but it would be a huge boost for those who do.

What alternatives to the feature have been considered?

Typing out every dependencies with my hands but it got tedious quite fast.

[MikeMcQuaid]

We should do this but it shouldn't be brew install A B C but instead brew fetch A B C; brew install A; brew install B; brew install C etc. for better/easier error-handling. This is similar to what brew test-bot does.