Consider adding shift+left extend selection as a default mouse mapping

[mcrmck]

Kitty already supports extending selections via right-click (ungrabbed) and shift+right (grabbed), and shift+left can be configured manually. This isn't a feature request -- the capability exists and works well.

The suggestion is a small change to the default mouse mappings:

# Current defaults:
mouse_map shift+left press ungrabbed,grabbed mouse_selection normal

# Proposed
mouse_map shift+left press grabbed mouse_selection normal
mouse_map shift+left press ungrabbed mouse_selection extend

In ungrabbed mode, plain left-click already starts a selection, so shift+left starting another new selection is redundant. Reassigning it to extend would match the default behavior of virtually every other terminal (iTerm2, Terminal.app, GNOME Terminal, Alacritty, Wezterm, Ghostty) and most GUI text applications (browsers, editors, file managers, etc.).

The grabbed-mode mapping stays unchanged -- shift+left still starts a new selection there, since plain left-click is captured by the application.

This came up in #8021 where a user was confused that shift+click didn't extend. The answer was "use right-click" which works, but the muiscle memory expectation is strong enough that it seems worth matching by default. Users who prefer the current behavior can override it one line.

Happy to submit a PR if this seems reasonable.

Cheers.

[mcrmck]

This was implemented in fcdc67a.

Thank you, as always @kovidgoyal.