rfc: use extmarks for highlighting and carets (#2099)

* Starting to make some progress on extmarks for prefixes

* Finished up extmarks for multi-icons

* Cleaned up update_prefix calls, added display highlights

* remove highlight_one_row

* Remove TODOs

* stylua fixes

* fixes for luacheck

* Fixed whitespace to appease the stylua gods

* fixed a couple nits from pr

* Got tests passing with new highlighting functionality

* Apply suggestions from code review

Co-authored-by: Fabian David Schmidt <39233597+fdschmidt93@users.noreply.github.com>

Co-authored-by: Fabian David Schmidt <39233597+fdschmidt93@users.noreply.github.com>

lua/tests/automated/pickers/find_files_spec.lua

@@ -26,7 +26,7 @@ describe("builtin.find_files", function()
         runner.picker('find_files', 'README.md', {
           post_typed = {
             { "> README.md", GetPrompt },
-            { "> README.md", GetBestResult },
+            { "  README.md", GetBestResult },
           },
           post_close = {
             { 'README.md', GetFile },
@@ -112,7 +112,7 @@ describe("builtin.find_files", function()
           {
             {
               "  lua/tests/fixtures/file_a.txt",
-              "> lua/tests/fixtures/file_abc.txt",
+              "  lua/tests/fixtures/file_abc.txt",
             }, GetResults
           },
         },