Rename main branch to 4.x and update installation instructions

[clue]

This changeset suggests renaming the development branch to 4.x and adds appropriate installation instructions. In particular, this makes it much easier to install the v4 development version or any version like this:

$ composer require react/async:^4@dev
$ composer require react/async:"^4@dev || ^3@dev || ^2@dev"

The same branch policy has been suggested for our Promise component via reactphp/promise#212. This is also in line with our existing 3.x and 2.x branches and means we do not have to update any alias definitions once we would start working on a potential future 5.x.

From a consumer's perspective, this means installation is now much easier, as targeting ^4@dev (or 4.x-dev) is semantically more obvious and safer than using dev-main. Additionally, keep in mind that branch names are development artifacts that are subject to change at some point in the future. Once we would decide to rename or delete any of our version branches, the ^4@dev reference would still be valid as it would also match stable versions if we have any ^4 tags at this point (which is safe to assume at this point).

Once this PR is merged, I will manually rename the main branch to 4.x. Unlike suggested in reactphp/promise#212, the default branch should not be changed. In other words, 4.x would be the new default branch, as both 3.x and 2.x are also still under active development.

Also refs #14 and #11 for 3.x and 2.x branch names already used in this project.