feat: Better border configuration.

Started adding `resolve` which will allow you to do some really nice
stuff easily with options to get the window options from a variety of
inputs.

You can see how we do it in pickers.lua for borders & borderchars
currently.

lua/tests/manual/resolver_spec.lua

@@ -0,0 +1,58 @@
+RELOAD('telescope')
+
+local resolve = require('telescope.config.resolve')
+
+local eq = function(a, b)
+  if a ~= b then
+    error(string.format(
+      "Expected a == b, got: %s and %s", vim.inspect(a), vim.inspect(b)
+    ))
+  end
+end
+
+local opt = nil
+
+local height_config = 0.8
+opt = resolve.win_option(height_config)
+
+eq(height_config, opt.preview)
+eq(height_config, opt.prompt)
+eq(height_config, opt.results)
+
+opt = resolve.win_option(nil, height_config)
+
+eq(height_config, opt.preview)
+eq(height_config, opt.prompt)
+eq(height_config, opt.results)
+
+local table_val = {'a'}
+opt = resolve.win_option(nil, table_val)
+eq(table_val, opt.preview)
+eq(table_val, opt.prompt)
+eq(table_val, opt.results)
+
+local prompt_override = {'a', prompt = 'b'}
+opt = resolve.win_option(prompt_override)
+eq('a', opt.preview)
+eq('a', opt.results)
+eq('b', opt.prompt)
+
+local all_specified = {preview = 'a', prompt = 'b', results = 'c'}
+opt = resolve.win_option(all_specified)
+eq('a', opt.preview)
+eq('b', opt.prompt)
+eq('c', opt.results)
+
+local some_specified = {prompt = 'b', results = 'c'}
+opt = resolve.win_option(some_specified, 'a')
+eq('a', opt.preview)
+eq('b', opt.prompt)
+eq('c', opt.results)
+
+-- local true_table = {true}
+-- opt = resolve.win_option(some_specified, 'a')
+-- eq('a', opt.preview)
+-- eq('b', opt.prompt)
+-- eq('c', opt.results)
+
+print("DONE!")