feat(codechef): implement full CodeChef support (#354)

## Problem

CodeChef had no working login, submit, or contest list. The browser
selectors were wrong, the contest list was missing present/past
contests,
and problem/contest URLs were unset.

## Solution

Fix login and submit selectors for the Drupal-based site. Paginate
`/api/list/contests/past` to collect all 228 Starters, then expand each
parent contest into individual division entries (e.g. `START228 (Div.
4)`).
Add language IDs, correct `url`/`contest_url`/`standings_url` in
metadata,
and make `:CP <platform>` open the contest picker directly.

lua/cp/init.lua

@@ -38,11 +38,13 @@ function M.handle_command(opts)
   commands.handle_command(opts)
 end
 
+---@return boolean
 function M.is_initialized()
   return initialized
 end
 
 ---@deprecated Use `vim.g.cp` instead
+---@param user_config table?
 function M.setup(user_config)
   vim.deprecate('require("cp").setup()', 'vim.g.cp', 'v0.7.7', 'cp.nvim', false)