Meta: Pretty-printing enhancements

[sleirsgoevy]

This fixes pretty-printing of AK::HashMap, and adds pretty-printing of AK::IntrusiveList

[BuggieBot]

Hello!

One or more of the commit messages in this PR do not match the SerenityOS code submission policy, please check the lint_commits CI job for more details on which commits were flagged and why.
Please do not close this PR and open another, instead modify your commit message(s) with git commit --amend and force push those changes to update this PR.

[nico]

Thanks! I don't use this file myself, so I'll trust you that the change works :) One stylistic comment below:

[nico]

Mention StringView in commit description too.

Actually, could you do one commit per thing?

• One for adding IntrusiveList
• One for fixing HashMap (add some more words on what exactly it fixes, see https://github.com/SerenityOS/serenity/blob/master/CONTRIBUTING.md Don't Use weasel-words like "refactor" or "fix" to avoid explaining what's being changed.)
• One for StringView, saying what exactly is changing here (I think this here is meant to be a no-op)

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions!

[stale]

This pull request has been closed because it has not had recent activity. Feel free to re-open if you wish to still contribute these changes. Thank you for your contributions!