Opening URLs via keyboard input

[pluiedev]

Successor to #2394.

URLs in Ghostty can currently only be accessed via Cmd/Ctrl+(Shift)+Clicking, which is not too conducive for people using primarily keyboard-based workflows. Other terminals (e.g. Kitty) and terminal apps (e.g. Vim, Neovim, Helix) use what's known as "hint mode" to generate a unique key combination for every URL present on the active screen, and the user can input that combination to access the URL.