visual editor: support multi-cursor 'add next match' in the find API

Context

RStudio's "Find and Add Next" and "Find All" commands build a multi-cursor selection from the current find match (Ace's "add next selection" feature). In the visual editor these commands work inside code chunks (routed to the chunk's embedded Ace editor, rstudio/rstudio#18651), but they cannot work in prose: the editor's find API only exposes single-selection operations, so RStudio currently reports the commands as disabled there.

Reported by users in rstudio/rstudio#16540, where the repro is specifically prose.

Feature request

An "add next match to selection" capability (and ideally a companion "select all matches") in the editor's find API, so embedders can offer multi-cursor style editing over find results in prose.

Technical notes

• EditorFindReplace (packages/editor/src/editor/editor.ts, exposed via Editor.getFindReplace()) currently exposes selectFirst / selectCurrent / selectNext / selectPrevious / replace / replaceAll, all single-selection, backed by the FindPlugin in packages/editor/src/behaviors/find.ts.
• ProseMirror's TextSelection is a single range and multi-range selection is not a native concept, so this likely requires either a custom Selection subclass or a plugin that maintains the additional ranges as decorations and fans typed edits out to every range.

Not a small change -- filing so the request is tracked upstream and can be prioritized. Happy to discuss design or contribute if there's an agreed direction.

Start with EditorFindReplace in packages/editor/src/editor/editor.ts and the FindPlugin in packages/editor/src/behaviors/find.ts. Review how the current single-selection operations work and investigate the ProseMirror TextSelection constraint before discussing an agreed design. Done means prose find results expose add-next-match, and ideally select-all, to embedders with multi-cursor editing behavior.