feat(design): add a tree component

[damienwebdev]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/graycoreio/daffodil/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Currently, we don't have any way to describe a tree of elements with an aesthetic and usable design.

Fixes: N/A

What is the new behavior?

This adds such a component.

We've also added considerations for "flat" trees, e.g. trees that visually look like trees, but semantically are merely flat lists.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[xelaint]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[griest024]

I think the Daffio changes should be a separate PR

[xelaint]

I think the Daffio changes should be a separate PR

@griest024 Split the PR, implementation is now in #1824

[griest024]

tree does not appear in the sidebar list. Navigating manually to /tree throws this error:

ERROR Error: Uncaught (in promise): NotSupportedError: Failed to execute 'define' on 'CustomElementRegistry': the name "basic-tree-example" has already been used with this registry