Add fish_mode_prompt.fish

Make mode prompt display correctly when using `fish_vi_keybindings`. Comparison: https://i.imgur.com/yNYkKV2.png

fish_mode_prompt.fish

@@ -0,0 +1,15 @@
+function fish_mode_prompt
+    if test "$fish_key_bindings" = "fish_vi_key_bindings"
+        or test "$fish_key_bindings" = "fish_hybrid_key_bindings"
+        switch $fish_bind_mode
+            case default
+                prompt_segment blue black "N"
+            case insert
+                prompt_segment green black "I"
+            case replace_one
+                prompt_segment red black "R"
+            case visual
+                prompt_segment magenta black "V"
+        end
+    end
+end