feat(commands): add :CP <platform> signup subcommand

Problem: No quick way to reach a platform's registration page from
within Neovim.

Solution: Add `signup` as a platform subcommand that calls `vim.ui.open`
on the platform's registration URL. Works even when the platform is
disabled in config. URLs live in a new `SIGNUP_URLS` table in
`constants.lua`. Tab completion and vimdoc updated.

lua/cp/constants.lua

@@ -25,6 +25,15 @@ M.PLATFORM_DISPLAY_NAMES = {
   usaco = 'USACO',
 }
 
+M.SIGNUP_URLS = {
+  atcoder = 'https://atcoder.jp/register',
+  codechef = 'https://www.codechef.com/register',
+  codeforces = 'https://codeforces.com/register',
+  cses = 'https://cses.fi/register',
+  kattis = 'https://open.kattis.com/register',
+  usaco = 'https://usaco.org/index.php?page=createaccount',
+}
+
 M.CPP = 'cpp'
 M.PYTHON = 'python'