fix: insert child right after the last after: match in add_new_child/3

[pshoukry]

add_new_child/3 with after: inserted the child one place too late whenever a non-matching child followed the last match. Closes #406.

skip_after/2 walked right past the last match and, on reaching a non-matching child, returned {:after, that_child} because of a nested? flag. The flag is gone: a non-matching child now returns {:before, child}, which is the slot right after the last match.

# children = [Test.Repo, TestWeb.Endpoint]
add_new_child(igniter, NewChild, after: [Test.Repo])
# before => [Test.Repo, TestWeb.Endpoint, NewChild]
# after  => [Test.Repo, NewChild, TestWeb.Endpoint]

Real change: lib/igniter/project/application.ex (+2/-6).

Tests (test/igniter/project/application_test.exs, 5 new):

Case	Without fix
match followed by a non-matching child	fails
several matches, then a non-matching child	fails
non-matching child between matches	fails
match is the last child	passes (control)
no match, child goes first	passes (control)

Full suite: 444 tests, 1 doctest, 0 failures. mix format --check-formatted, mix credo --strict and mix dialyzer are clean.

I did not touch CHANGELOG.md; the release task generates it from commit messages.

[zachdaniel]

🚀 Thank you for your contribution! 🚀