Tree selector on the map

We need a selector feature for the web map core, it can do:
* Pick a single tree on the map
* A panel or drawer to list all trees selected
* A api or function or event to get the list of tree (so the client can get it)

Advanced feature:
* Draw a polygon on the map to select a bunch of trees.

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Some hints:

- Please read our [readme](https://github.com/Greenstand/treetracker-web-map-core#treetracker-web-map-core) for more information/guide/tutorial.
- Here is [an engineering book](https://greenstand.gitbook.io/engineering/) in Greenstand.
- To know more about our organization, visit our [website](https://greenstand.org).
- If you want to join the slack community (some resources need the community member's permission), please leave your email address.

Start with the repository README and its web map core usage guide. Clarify the scope and API for single-tree selection, listing selected trees, and exposing the selected list before considering polygon selection. Done should include the agreed selector behavior and a client-accessible list of selected trees.

Written by the indexing model from the issue text.