`dep add`: avoid long lines in `:aliases`

[teodorlu]

Please answer the following questions and leave the below in as part of your PR.

• [x] This PR corresponds to an issue with a clear problem statement.

  • https://github.com/babashka/neil/issues/224

• [x] I have updated the CHANGELOG.md file with a description of the addressed issue.

  • I consider this work to be covered by the changelog entry for https://github.com/babashka/neil/issues/215

• [x] I have considered whether I should add more tests covering the code I've changed.

  • New tests have been added for the new function babashka.neil/value->assoc-in-pairs, and a test for babashka.neil/add-alias-str, covering the case where the :nrepl alias produced very long lines in :extra-deps

Behavior before this PR

Given this deps.edn

{:aliases
 {:dev {}}}

, neil dep add nrepl would produce this deps.edn:

{:aliases
 {:dev {}
  :nrepl {:extra-deps {nrepl/nrepl {:mvn/version "1.1.2"} cider/cider-nrepl {:mvn/version "0.49.0"} refactor-nrepl/refactor-nrepl {:mvn/version "3.10.0"}}
          :main-opts ["-m" "nrepl.cmdline" "--interactive" "--color" "--middleware" "[cider.nrepl/cider-middleware,refactor-nrepl.middleware/wrap-refactor]"]}}}

Behavior after this PR

With the same deps.edn, neil dep add nrepl would produce this deps.edn:

{:aliases
 {:dev {}
  :nrepl {:extra-deps {nrepl/nrepl {:mvn/version "1.1.2"}
                       cider/cider-nrepl {:mvn/version "0.49.0"}
                       refactor-nrepl/refactor-nrepl {:mvn/version "3.10.0"}}
          :main-opts ["-m" "nrepl.cmdline" "--interactive" "--color" "--middleware" "[cider.nrepl/cider-middleware,refactor-nrepl.middleware/wrap-refactor]"]}}}

[teodorlu]

I think I'm happy with the code now.