feat(input): select the next occurrence, or all of them, as more selections

[trancong12102]

Adds ⌘D-style occurrence selection to the multi-line inputs (code editor and textarea), built on the existing multi-cursor selections. No upstream PR covers it (checked longbridge/gpui-component's open PRs on 2026-10-01).

• With only carets, the first press selects the word under the newest caret (or the word just before it when the caret sits at its end).
• SelectNextOccurrence then adds the next occurrence of the newest selection's text as another selection, wrapping past the end and skipping text already selected, and scrolls to it.
• SelectAllOccurrences adds every occurrence.
• Neither has a default key binding; applications bind them (Slopty binds ⌘D and ⌘⇧L in its file editor).

Public API

gpui-base (re-exported from gpui-component's input):

• actions::SelectNextOccurrence: select the word, then add the next occurrence as a selection.
• actions::SelectAllOccurrences: select the word, then every occurrence.
• pub use lsp_types; in gpui_base::input: the LSP types the providers speak, so a CompletionProvider implementer needs no lsp-types dependency pinned to the same version.

Tests: input::occurrences::tests (word first then wrap, all at once, a caret off any word). cargo clippy -p gpui-base fails on main already with too_many_arguments in dock_area.rs and text_wrapper.rs, untouched here.

Co-Authored-By: Claude Code