feat: Themes (#79)

Big shoutout to @rockerBOO  for the idea and basically all the impl!

* feat: Add dropdown layout strategy

* Adding more docs. Working better with options

* Refactor borders

* Fix for spaces

* Add preview to layout. Add Themes.

* feat: themes

Co-authored-by: TJ DeVries <devries.timothyj@gmail.com>

lua/telescope/themes.lua

@@ -0,0 +1,29 @@
+-- Prototype Theme System (WIP)
+-- Currently certain designs need a number of parameters.
+--
+-- local opts = themes.get_dropdown { winblend = 3 }
+--
+
+local themes = {}
+
+function themes.get_dropdown(opts)
+  local theme_opts = {
+    -- WIP: Decide on keeping these names or not.
+    theme = "dropdown",
+
+    sorting_strategy = "ascending",
+    layout_strategy = "center",
+    results_title = false,
+    preview_title = "Preview",
+    border = false,
+    borderchars = {
+      prompt = {"─", "│", " ", "│", "╭", "╮", "│", "│"},
+      results = {"─", "│", "─", "│", "├", "┤", "╯", "╰"},
+      preview = {"=", "=", "", "", "", "", "", ""}
+    },
+  }
+
+  return vim.tbl_deep_extend("force", theme_opts, opts)
+end
+
+return themes