text: extend TextView selection on shift-click (#2729)

## Summary

- preserve the latest plain-click anchor when extending TextView
selection with Shift+click
- support repeated Shift+click, reversed extension, Shift+drag, and
cross-TextView selection
- keep suppression and selection lifecycle behavior safe across
interactive controls
- document the AppKit, local Editor, and pinned Zed behavior used to
choose the design

## Test Plan

- `cargo test -p gpui-component --lib`
- `cargo check -p gpui-component`
- `cargo fmt --all -- --check`
- `git diff --check`

Co-authored-by: Codex <codex@openai.com>