feat(commands): implement :CP open [problem|contest|standings] (#319)

## Problem

There was no way to open a problem, contest, or standings page in the
browser from within the plugin.

## Solution

Add `contest_url` and `standings_url` to `MetadataResult` and persist
them in the cache. Add `cache.get_open_urls` to resolve all three URLs.
Wire up `:CP open [problem|contest|standings]` in `commands/init.lua`
to call `vim.ui.open`, warning when a URL is unavailable (e.g. CSES
has no standings). Closes #315.

lua/cp/constants.lua

@@ -13,6 +13,7 @@ M.ACTIONS = {
   'race',
   'stress',
   'submit',
+  'open',
 }
 
 M.PLATFORM_DISPLAY_NAMES = {