Add theme key for selected line number

Adds `ui.linenr.selected` which controls highlight of linu numbes which
have cursors on.

- Fallback to linenr if linenr.selected is missing

- Update docs and themes

- Add TODOs for themes with temporary linenr.selected

contrib/themes/bogster.toml

@@ -30,6 +30,7 @@
 
 "ui.background" = { bg = "#161c23" }
 "ui.linenr" = { fg = "#415367" }
+"ui.linenr.selected" = { fg = "#e5ded6" }  # TODO
 "ui.statusline" = { bg = "#232d38" }
 "ui.popup" = { bg = "#232d38" }
 "ui.window" = { bg = "#232d38" }

contrib/themes/ingrid.toml

@@ -30,6 +30,7 @@
 
 "ui.background" = { bg = "#FFFCFD" }
 "ui.linenr" = { fg = "#bbbbbb" }
+"ui.linenr.selected" = { fg = "#F3EAE9" }  # TODO
 "ui.statusline" = { bg = "#F3EAE9" }
 "ui.popup" = { bg = "#F3EAE9" }
 "ui.window" = { bg = "#D8B8B3" }

contrib/themes/onedark.toml

@@ -33,6 +33,7 @@
 "ui.background" = { fg = "#ABB2BF", bg = "#282C34" }
 "ui.help" = { bg = "#3E4452" }
 "ui.linenr" = { fg = "#4B5263", modifiers = ['dim'] }
+"ui.linenr.selected" = { fg = "#ABB2BF" }
 "ui.popup" = { bg = "#3E4452" }
 "ui.statusline" = { fg = "#ABB2BF", bg = "#2C323C" }
 "ui.selection" = { bg = "#3E4452" }