The components repo does not use the autosquash merge strategy, so
recent changes to that seem to broke the autosquash strategy.
Since we don't run the rebase in interactive mode, the `--autosquash`
flag has no effect. This is by design in Git. We can make it work by
setting the git sequence editor to `true` so that the rebase seems
like an interactive one to Git, while it isn't one for the user.
This matches conceptually with the merge script currently used in
framework. The only difference is that we allow a real interactive
rebase if the `commit message fixup` label is applied. This allows
commit message modifications (and others) if needed.
PR Close#37138
Moves the merge script from the components repository over
to the shared dev-infra package. The merge script has been
orginally built for all Angular repositories, but we just
kept it in the components repo temporarily to test it.
Since everything went well on the components side, we now
move the script over and integrate it into the dev-infra package.
PR Close#37138
