feat(Tree): added selectable prop

[epessina]

Description

Tree

- Added `selectable` property

Checklist

• [x] commit message and branch name follow conventions
• [x] tests are included
• [x] changes are accessible and documented from components stories
• [x] typings are updated or integrated accordingly with your changes
• [x] all added components are exported from index file (if necessary)
• [x] all added files include Apache 2.0 license
• [x] you are not committing extraneous files or sensitive data
• [x] the browser console does not have any logged errors
• [x] necessary labels have been applied to this pull request (enhancement, bug, ecc.)

[GiovannaMonti]

Sorry, did not see the PR. We had the same need and addressed it here: https://github.com/mia-platform/design-system/pull/569 If you agree, we can close this one.