fix: a lot of small things and adds more customization for caret (#554)

Attention:
prompt_prefix will no longer add a space at the end. So if you still want a space at the end make sure your configuration has one.
The default should not be changed. So if you haven't copied prompt_prefix in your config this doesn't affect you.

Feat:
- prompt prefix does no longer always end with space
- selection_caret configurable. Default: `> `
- result_prefix configurable. Default: `  `
- more actions for git_branches
  - <c-t> does track the branch
  - <c-r> does rebase branch
  - also added delete branch action but not configured. See readme on how to do that

Fixes:
- fix docgen ci
- Better error for lsp_workspace_symbols
- better formatting for CONTRIBUTING.md
- move from systemlist to plenary.job
- git branch now supports checkout on remote branches

lua/telescope/utils.lua

@@ -199,7 +199,11 @@ function utils.get_os_command_output(cmd, cwd)
     return {}
   end
   local command = table.remove(cmd, 1)
-  return Job:new({ command = command, args = cmd, cwd = cwd }):sync()
+  local stderr = {}
+  local stdout, ret = Job:new({ command = command, args = cmd, cwd = cwd, on_stderr = function(_, data)
+    table.insert(stderr, data)
+  end }):sync()
+  return stdout, ret, stderr
 end
 
 utils.strdisplaywidth = (function()