Adapt positioning of the inline chat

[aiday-mar]

Fixes: microsoft/vscode-copilot-release#52
Fixes: https://github.com/microsoft/vscode-copilot/issues/251

When the inline chat appears, it is positioned at the primary cursor. When markdown messages or errors are rendered, the inline chat position remains at the initial position. It only changes position if there has been an edit response and therefore the editor model has changed. The positioning is as follows and depends on the viewing strategy for the inline chat:

• livePreview: the inline chat is positioned at the end of the selection
• live: the inline chat is positioned at the line number corresponding to the last local edit, or at the initial position if there are no local edits
• preview: the inline chat is positioned at the initial position. Since no actual edits are done, it does not need to be positioned elsewhere.