See bevyengine/bevy#4078.

It would be nice if we could support the development of multi-threading features for web builds, as that could yield massive performance improvements.

Currently, contributors (and eventually, users) have to manually set environment flags and server headers. This creates a higher barrier to contributing to this development.
We should investigate whether we can implement some of these in the CLI to reduce the setup time and simplify contribution.

Depending on the type of change, we can either have it always active, or add a --multi-threaded flag to conditionally enable it.

Note that the changes shouldn't require changes in the Bevy application as well to make them work. This would break the CLI for non-multi-threaded apps.