Configuration for undercurl vertical offset

[smprather]

Undercurl is colliding with the bottom of the font. Requesting a configuration to apply a y-offset from the default vertical placement.

Here are some other-term links that may prevent having to think it through from scratch. https://github.com/kovidgoyal/kitty/issues/853 https://github.com/microsoft/cascadia-code/issues/395 https://wezfurlong.org/wezterm/config/lua/config/underline_position.html