Add Text2d picking backend

[coreh]

Objective

• This PR adds support for picking Text2d.

Solution

• Introduce a new feature flag, bevy_text2d_picking_backend, and add all the required plumbing;
• Extend the TextPlugin struct to include a add_picking setting;
• Port over the existing sprite picking backend to bevy_text, using TextLayoutInfo for the bounds;
• Add a text2d_picking example.

Note: I'm deliberately not calling the feature bevy_text_picking_backend, to avoid misleading people into thinking this might be used for UI Text.

Testing

• Did you test these changes? If so, how? Yes, by running the text2d_picking example.
• Are there any parts that need more testing? Not that I'm aware of
• How can other people (reviewers) test your changes? Is there anything specific they need to know? Run the text2d_picking example.
• If relevant, what platforms did you test these changes on, and are there any important ones you can't test? macOS

[github-actions]

The generated examples/README.md is out of sync with the example metadata in Cargo.toml or the example readme template. Please run cargo run -p build-templated-pages -- update examples to update it, and commit the file change.

[github-actions]

You added a new feature but didn't update the readme. Please run cargo run -p build-templated-pages -- update features to update it, and commit the file change.