`neil add kaocha`: normalize alias indentation

[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/215

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

Problem

This PR addresses #215. Currently, neil add kaocha may add the :kaocha alias with one fewer indent than expected, leaving a deps.edn file like this:

{:paths ["src"]
 :deps {babashka/fs {:mvn/version "0.2.16"}
        babashka/process {:mvn/version "0.4.16"}
        borkdude/edamame {:mvn/version "1.4.25"}
        cheshire/cheshire {:mvn/version "5.11.0"}
        enlive/enlive {:mvn/version "1.1.6"}
        org.babashka/cli {:mvn/version "0.6.44"}
        org.clj-commons/hickory {:mvn/version "0.7.3"}
        org.clojure/data.xml {:mvn/version "0.0.8"}}
 :aliases
 {:dev
  {:extra-paths ["dev" "test"]
   :extra-deps {io.github.nextjournal/clerk {:mvn/version "0.13.842"}}}

 :kaocha ;; added by neil
 {:extra-deps {lambdaisland/kaocha {:mvn/version "1.91.1392"}}
  :main-opts ["-m" "kaocha.runner"]}}}

The :dev and :kaocha aliases don't line up.

Proposed solution

• Split add-alias into a pure part (add-alias-str) and an impure part (add-alias)
• Implement add-alias-str using only borkdude.rewrite-edn/assoc-in, avoid manually calculating indent spaces

Tests

I've tried to write the tests so that it's easy to see visually how indents line up.

Future work

If we merge this PR, we may consider storing kaocha-alias and nrepl-alias as clojure data. Currently, they are strings:

https://github.com/teodorlu/neil/blob/02c0fc9c68db2e9f0cd876605eb1aca6ab7a013f/src/babashka/neil.clj#L205-L209

https://github.com/teodorlu/neil/blob/02c0fc9c68db2e9f0cd876605eb1aca6ab7a013f/src/babashka/neil.clj#L225-L229

[borkdude]

Thanks!