Support `chat.editor.wordSeparators` setting for GitHub Copilot Chat input boxes

In the VS Code editor, `Ctrl + ←/→` moves the cursor according to the `editor.wordSeparators` setting. For example, if a user adds Chinese punctuation (such as `。`) to `editor.wordSeparators`, pressing `Ctrl + ←/→` in the editor will jump over an entire Chinese sentence until the next punctuation mark.

However, the Copilot Chat input box does not provide a way to customize word separators. As a result, continuous CJK text (including punctuation) is treated as a single word. When CJK users use `Ctrl + ←/→` to move the cursor, it always jumps straight over an entire paragraph. This makes editing and navigating within the chat input extremely slow and frustrating.

I propose adding a dedicated setting, e.g. `chat.editor.wordSeparators`, to control word boundaries specifically in the Copilot Chat input box. This setting would work independently from `editor.wordSeparators`, allowing the editor and the chat input to use different separator rules. This feature would greatly improve the editing experience for CJK users and keep the chat input configuration separate from the code editor configuration.