Fixes #580. Mark Terminal.Gui.TextView as [Obsolete] in favor of gui-cs/Editor

[Copilot]

Fixes

• Fixes Fixing many clipping issues #580

Proposed Changes/Todos

Adds soft [Obsolete(error: false)] to TextView and types that only exist in a TextView context, steering consumers toward gui-cs/Editor.

Marked obsolete:

• TextView — with message listing Editor's advantages (rope-backed document, multi-caret, syntax highlighting, etc.)
• TextViewAutocomplete — only meaningful with TextView
• ContentsChangedEventArgs — only used by TextView.ContentsChanged

Not marked obsolete (intentionally):

• TextModel, WordWrapManager, HistoryText — internal, invisible to consumers
• HistoryTextItemEventArgs, TextEditingLineStatus — shared with TextField

[Obsolete ("TextView is superseded by gui-cs/Editor's EditorView, which provides a rope-backed document model, "
           + "cell-aware rendering, multi-caret editing, undo, syntax highlighting, folding, find/replace, and soft wrap. "
           + "See https://github.com/gui-cs/Editor for details.", error: false)]
public partial class TextView : View, IDesignable { ... }

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working