Add new prop to configure tree node selectable

[lyy011lyy]

Fixes #

In some cases, we don't want the tree node to be selectable and keep refreshing itself, the issue description: #3081

So I'm thinking to introduce this new prop for the Tree component to make the selectable node configurable.

Additional description

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[interactivellama]

@lyy011lyy Hi, I've been on work leave until this week. Elements with role treeitems were never meant to have clickable children which is what the screen capture looks like. Per ARIA Tree spec, there's only one focus at a time in a Tree widget https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles/tree_role#single_and_multiple_select_trees

I haven't looked over the PR yet, but am discussing what the Definition of Done for handling keyboard only use would look like in a Tree pattern that confirms to the ARIA spec, and I'm having difficulty with it. This library implements the patterns here: https://www.lightningdesignsystem.com/components/trees/ which are the scope of this component.

Based on the screenshot, you may be helped by the Tree Grid pattern which does allow multiple focusable elements.
https://www.lightningdesignsystem.com/components/tree-grid/#Single-select-headless-with-no-borders-and-a-selected-row

A full TreeGrid pattern has not been added to this library. It would be great to add it. Here is a branch that is close to what you are looking for with a TreeGrid pattern + dropdown menu column. #2181

The interactions and accessibility were never completed though however in order to add it officially to this library.