Modularize theme selection for whichkey's telescope commands

[ClashTheBunny]

Currently, each whichkey is configured slightly differently based on the
type of theme used for it. This makes them all structured the same so
that it's easier to switch between dropdown or ivy. The nice thing is
that one can then replace ivy{} with dropdown{} more easily.

[ClashTheBunny]

An example would be if you wanted consistency for live_grep vs the other usage of telescope:

❯ git diff                                                                                                                                                               [22:57:27]
diff --git i/lua/user/whichkey.lua w/lua/user/whichkey.lua
index 6e784ea..053b221 100644
--- i/lua/user/whichkey.lua
+++ w/lua/user/whichkey.lua
@@ -80,7 +80,7 @@ local opts = {

 local buffer_theme = "dropdown{previewer = false}"
 local find_file_theme = "dropdown{previewer = false}"
-local find_text_theme = "ivy{}"
+local find_text_theme = "dropdown{}"

 local mappings = {
   ["a"] = { "<cmd>Alpha<cr>", "Alpha" },