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/tui/mode/mini/underline.zig

@@ -0,0 +1,17 @@
+const std = @import("std");
+const cbor = @import("cbor");
+const command = @import("command");
+
+const tui = @import("../../tui.zig");
+
+pub const Type = @import("get_char.zig").Create(@This());
+pub const create = Type.create;
+
+pub fn name(_: *Type) []const u8 {
+    return "underline";
+}
+
+pub fn process_egc(_: *Type, egc: []const u8) command.Result {
+    try command.executeName("underline_with_char", command.fmt(.{ egc, "solid" }));
+    try command.executeName("exit_mini_mode", .{});
+}