feat: add underlining keybinds ctrl+_, ctrl+= and ctrl+plus

ctrl+_ => select char to underline with
ctrl+= => underline with '='
ctrl+plus => underline with '=' preserving spaces

closes #350

src/keybind/builtin/flow.json

@@ -136,6 +136,9 @@
             ["alt+u", "to_upper"],
             ["alt+l", "to_lower"],
             ["alt+c", "switch_case"],
+            ["ctrl+_", "underline"],
+            ["ctrl+=", "underline_with_char", "=", "solid"],
+            ["ctrl+plus", "underline_with_char", "="],
             ["alt+b", "move_word_left"],
             ["alt+f", "move_word_right"],
             ["alt+s", "filter", "sort"],