input: Add InputState::add_selection for programmatic multi-cursor

[sola-ryu]

Multi-cursor editing exists internally (Alt-click, the AddCursorAbove/Below actions), but applications had no public API to place a cursor at an arbitrary byte range. That makes features like "select all occurrences" (VS Code's Cmd+Shift+L) or additive "select next occurrence" impossible to build on top of the editor.

This change adds a single public method, InputBaseState::add_selection, the programmatic equivalent of Alt-clicking another cursor into the text.

Behavior mirrors the existing internal paths:

• The range is clipped to the text and expanded to character boundaries, like set_selected_range.
• A range already covered by a selection adds nothing (Alt-click is idempotent the same way).
• On a single-line input there is no second cursor, so the call falls back to replacing the selection.
• The view scrolls to the new selection, matching set_selected_range via move_to.

Public API

gpui-base

• InputBaseState::add_selection(&mut self, range: Range<usize>, cx: &mut Context<Self>) — Adds range as an additional selection while keeping existing ones, enabling programmatic multi-cursor placement (e.g. select-all-occurrences).

---

This PR is 100% AI-generated, reviewed and validated by the contributor: cargo fmt --check, cargo clippy -p gpui-base --all-targets -- --deny warnings, and the input test suite (329 passed, including the new test_add_selection) are all green.