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/constants.lua

@@ -208,6 +208,12 @@ M.LANGUAGE_VERSIONS = {
     python = { python3 = 'PYTH 3', pypy3 = 'PYPY3' },
     java = { java = 'JAVA' },
     rust = { rust = 'rust' },
+    c = { c = 'C' },
+    go = { go = 'GO' },
+    kotlin = { kotlin = 'KTLN' },
+    javascript = { nodejs = 'NODEJS' },
+    typescript = { typescript = 'TS' },
+    csharp = { csharp = 'C#' },
   },
 }