Modularize theme selection for whichkey's telescope commands

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.

lua/user/whichkey.lua

@@ -78,10 +78,14 @@ local opts = {
   nowait = true, -- use `nowait` when creating keymaps
 }
 
+local buffer_theme = "dropdown{previewer = false}"
+local find_file_theme = "dropdown{previewer = false}"
+local find_text_theme = "ivy{}"
+
 local mappings = {
   ["a"] = { "<cmd>Alpha<cr>", "Alpha" },
   ["b"] = {
-    "<cmd>lua require('telescope.builtin').buffers(require('telescope.themes').get_dropdown{previewer = false})<cr>",
+    "<cmd>lua require('telescope.builtin').buffers(require('telescope.themes').get_" .. buffer_theme .. ")<cr>",
     "Buffers",
   },
   ["e"] = { "<cmd>NvimTreeToggle<cr>", "Explorer" },
@@ -90,10 +94,13 @@ local mappings = {
   ["c"] = { "<cmd>Bdelete!<CR>", "Close Buffer" },
   ["h"] = { "<cmd>nohlsearch<CR>", "No Highlight" },
   ["f"] = {
-    "<cmd>lua require('telescope.builtin').find_files(require('telescope.themes').get_dropdown{previewer = false})<cr>",
+    "<cmd>lua require('telescope.builtin').find_files(require('telescope.themes').get_" .. find_file_theme .. ")<cr>",
     "Find files",
   },
-  ["F"] = { "<cmd>Telescope live_grep theme=ivy<cr>", "Find Text" },
+  ["F"] = {
+    "<cmd>lua require('telescope.builtin').live_grep(require('telescope.themes').get_" .. find_text_theme .. ")<cr>",
+    "Find text",
+  },
   ["P"] = { "<cmd>lua require('telescope').extensions.projects.projects()<cr>", "Projects" },
 
   p = {