feat(commands): add :CP <platform> signup subcommand (#337)

## 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. URLs live in a new `SIGNUP_URLS` table in
`constants.lua`. Works even when the platform is disabled. 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'
 

plugin/cp.lua

@@ -64,7 +64,7 @@ end, {
       return filter_candidates(candidates)
     elseif num_args == 3 then
       if vim.tbl_contains(platforms, args[2]) then
-        local candidates = { 'login', 'logout' }
+        local candidates = { 'login', 'logout', 'signup' }
         local cache = require('cp.cache')
         cache.load()
         vim.list_extend(candidates, cache.get_cached_contest_ids(args[2]))